Wastewater Characteristics, Treatment and Disposal

Biological Wastewater Treatment Series The Biological Wastewater Treatment series is based on the book Biological Wastewater Treatment in Warm Climate Regions and on a highly acclaimed set of best selling textbooks. This international version is comprised by six textbooks giving a state-of-the-art presentation of the science and technology of biological wastewater treatment. Titles in the Biological Wastewater Treatment series are: Volume 1: Wastewater Characteristics, Treatment and Disposal Volume 2: Basic Principles of Wastewater Treatment Volume 3: Waste Stabilisation Ponds Volume 4: Anaerobic Reactors Volume 5: Activated Sludge and Aerobic Biofilm Reactors Volume 6: Sludge Treatment and Disposal

Biological Wastewater Treatment Series VOLUME ONE

Wastewater Characteristics, Treatment and Disposal Marcos von Sperling Department of Sanitary and Environmental Engineering Federal University of Minas Gerais, Brazil

Published by IWA Publishing, Alliance House, 12 Caxton Street, London SW1H 0QS, UK Telephone: +44 (0) 20 7654 5500; Fax: +44 (0) 20 7654 5555; Email: [email protected] Website: www.iwapublishing.com First published 2007
C 2007 IWA Publishing Copy-edited and typeset by Aptara Inc., New Delhi, India Printed by Lightning Source Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the UK Copyright, Designs and Patents Act (1998), no part of this publication may be reproduced, stored or transmitted in any form or by any means, without the prior permission in writing of the publisher, or, in the case of photographic reproduction, in accordance with the terms of licences issued by the Copyright Licensing Agency in the UK, or in accordance with the terms of licenses issued by the appropriate reproduction rights organization outside the UK. Enquiries concerning reproduction outside the terms stated here should be sent to IWA Publishing at the address printed above. The publisher makes no representation, expressed or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for errors or omissions that may be made. Disclaimer The information provided and the opinions given in this publication are not necessarily those of IWA or of the editors, and should not be acted upon without independent consideration and professional advice. IWA and the editors will not accept responsibility for any loss or damage suffered by any person acting or refraining from acting upon any material contained in this publication. British Library Cataloguing in Publication Data A CIP catalogue record for this book is available from the British Library Library of Congress Cataloguing-in-Publication Data A catalogue record for this book is available from the Library of Congress

ISBN: 1 84339 161 9 ISBN 13: 9781843391616

Contents

Preface The author

vii xi

1 Introduction to water quality and water pollution 1.1 Introduction 1.2 Uses of water 1.3 Water quality requirements 1.4 Water pollution 2 Wastewater characteristics 2.1 Wastewater flowrates 2.2 Wastewater composition

1 1 3 3 6 9 9 28

3 Impact of wastewater discharges to water bodies 3.1 Introduction 3.2 Pollution by organic matter and stream self purification 3.3 Contamination by pathogenic microorganisms 3.4 Eutrophication of lakes and reservoirs 3.5 Quality standards for wastewater discharges and waterbodies

77 77 77 123 131 150

4 Overview of wastewater treatment systems 4.1 Wastewater treatment levels 4.2 Wastewater treatment operations, processes and systems 4.3 Preliminary treatment 4.4 Primary treatment 4.5 Secondary treatment 4.6 Removal of pathogenic organisms 4.7 Analysis and selection of the wastewater treatment process

163 163 165 178 179 180 215 215

v

vi

Contents

5 Overview of sludge treatment and disposal 5.1 Introduction 5.2 Relationships in sludge: solids levels, concentration and flow 5.3 Quantity of sludge generated in the wastewater treatment processes 5.4 Sludge treatment stages 5.5 Sludge thickening 5.6 Sludge stabilisation 5.7 Sludge dewatering 5.8 Sludge disinfection 5.9 Final disposal of the sludge

242 242

6 Complementary items in planning studies 6.1 Preliminary studies 6.2 Design horizon and staging periods 6.3 Preliminary design of the alternatives 6.4 Economical study of alternatives

277 277 279 281 282

References

246 249 252 255 256 258 268 272

287

Preface

The present series of books has been produced based on the book “Biological wastewater treatment in warm climate regions”, written by the same authors and also published by IWA Publishing. The main idea behind this series is the subdivision of the original book into smaller books, which could be more easily purchased and used. The implementation of wastewater treatment plants has been so far a challenge for most countries. Economical resources, political will, institutional strength and cultural background are important elements defining the trajectory of pollution control in many countries. Technological aspects are sometimes mentioned as being one of the reasons hindering further developments. However, as shown in this series of books, the vast array of available processes for the treatment of wastewater should be seen as an incentive, allowing the selection of the most appropriate solution in technical and economical terms for each community or catchment area. For almost all combinations of requirements in terms of effluent quality, land availability, construction and running costs, mechanisation level and operational simplicity there will be one or more suitable treatment processes. Biological wastewater treatment is very much influenced by climate. Temperature plays a decisive role in some treatment processes, especially the natural-based and non-mechanised ones. Warm temperatures decrease land requirements, enhance conversion processes, increase removal efficiencies and make the utilisation of some treatment processes feasible. Some treatment processes, such as anaerobic reactors, may be utilised for diluted wastewater, such as domestic sewage, only in warm climate areas. Other processes, such as stabilisation ponds, may be applied in lower temperature regions, but occupying much larger areas and being subjected to a decrease in performance during winter. Other processes, such as activated sludge and aerobic biofilm reactors, are less dependent on temperature,

vii

viii

Preface

as a result of the higher technological input and mechanisation level. The main purpose of this series of books is to present the technologies for urban wastewater treatment as applied to the specific condition of warm temperature, with the related implications in terms of design and operation. There is no strict definition for the range of temperatures that fall into this category, since the books always present how to correct parameters, rates and coefficients for different temperatures. In this sense, subtropical and even temperate climate are also indirectly covered, although most of the focus lies on the tropical climate. Another important point is that most warm climate regions are situated in developing countries. Therefore, the books cast a special view on the reality of these countries, in which simple, economical and sustainable solutions are strongly demanded. All technologies presented in the books may be applied in developing countries, but of course they imply different requirements in terms of energy, equipment and operational skills. Whenever possible, simple solutions, approaches and technologies are presented and recommended. Considering the difficulty in covering all different alternatives for wastewater collection, the books concentrate on off-site solutions, implying collection and transportation of the wastewater to treatment plants. No off-site solutions, such as latrines and septic tanks are analysed. Also, stronger focus is given to separate sewerage systems, although the basic concepts are still applicable to combined and mixed systems, especially under dry weather conditions. Furthermore, emphasis is given to urban wastewater, that is, mainly domestic sewage plus some additional small contribution from non-domestic sources, such as industries. Hence, the books are not directed specifically to industrial wastewater treatment, given the specificities of this type of effluent. Another specific view of the books is that they detail biological treatment processes. No physical-chemical wastewater treatment processes are covered, although some physical operations, such as sedimentation and aeration, are dealt with since they are an integral part of some biological treatment processes. The books’ proposal is to present in a balanced way theory and practice of wastewater treatment, so that a conscious selection, design and operation of the wastewater treatment process may be practised. Theory is considered essential for the understanding of the working principles of wastewater treatment. Practice is associated to the direct application of the concepts for conception, design and operation. In order to ensure the practical and didactic view of the series, 371 illustrations, 322 summary tables and 117 examples are included. All major wastewater treatment processes are covered by full and interlinked design examples which are built up throughout the series and the books, from the determination of the wastewater characteristics, the impact of the discharge into rivers and lakes, the design of several wastewater treatment processes and the design of the sludge treatment and disposal units. The series is comprised by the following books, namely: (1) Wastewater characteristics, treatment and disposal; (2) Basic principles of wastewater treatment; (3) Waste stabilisation ponds; (4) Anaerobic reactors; (5) Activated sludge and aerobic biofilm reactors; (6) Sludge treatment and disposal.

Preface

ix

Volume 1 (Wastewater characteristics, treatment and disposal) presents an integrated view of water quality and wastewater treatment, analysing wastewater characteristics (flow and major constituents), the impact of the discharge into receiving water bodies and a general overview of wastewater treatment and sludge treatment and disposal. Volume 1 is more introductory, and may be used as teaching material for undergraduate courses in Civil Engineering, Environmental Engineering, Environmental Sciences and related courses. Volume 2 (Basic principles of wastewater treatment) is also introductory, but at a higher level of detailing. The core of this book is the unit operations and processes associated with biological wastewater treatment. The major topics covered are: microbiology and ecology of wastewater treatment; reaction kinetics and reactor hydraulics; conversion of organic and inorganic matter; sedimentation; aeration. Volume 2 may be used as part of postgraduate courses in Civil Engineering, Environmental Engineering, Environmental Sciences and related courses, either as part of disciplines on wastewater treatment or unit operations and processes. Volumes 3 to 5 are the central part of the series, being structured according to the major wastewater treatment processes (waste stabilisation ponds, anaerobic reactors, activated sludge and aerobic biofilm reactors). In each volume, all major process technologies and variants are fully covered, including main concepts, working principles, expected removal efficiencies, design criteria, design examples, construction aspects and operational guidelines. Similarly to Volume 2, volumes 3 to 5 can be used in postgraduate courses in Civil Engineering, Environmental Engineering, Environmental Sciences and related courses. Volume 6 (Sludge treatment and disposal) covers in detail sludge characteristics, production, treatment (thickening, dewatering, stabilisation, pathogens removal) and disposal (land application for agricultural purposes, sanitary landfills, landfarming and other methods). Environmental and public health issues are fully described. Possible academic uses for this part are same as those from volumes 3 to 5. Besides being used as textbooks at academic institutions, it is believed that the series may be an important reference for practising professionals, such as engineers, biologists, chemists and environmental scientists, acting in consulting companies, water authorities and environmental agencies. The present series is based on a consolidated, integrated and updated version of a series of six books written by the authors in Brazil, covering the topics presented in the current book, with the same concern for didactic approach and balance between theory and practice. The large success of the Brazilian books, used at most graduate and post-graduate courses at Brazilian universities, besides consulting companies and water and environmental agencies, was the driving force for the preparation of this international version. In this version, the books aim at presenting consolidated technology based on worldwide experience available at the international literature. However, it should be recognised that a significant input comes from the Brazilian experience, considering the background and working practice of all authors. Brazil is a large country

x

Preface

with many geographical, climatic, economical, social and cultural contrasts, reflecting well the reality encountered in many countries in the world. Besides, it should be mentioned that Brazil is currently one of the leading countries in the world on the application of anaerobic technology to domestic sewage treatment, and in the post-treatment of anaerobic effluents. Regarding this point, the authors would like to show their recognition for the Brazilian Research Programme on Basic Sanitation (PROSAB), which, through several years of intensive, applied, cooperative research has led to the consolidation of anaerobic treatment and aerobic/anaerobic post-treatment, which are currently widely applied in full-scale plants in Brazil. Consolidated results achieved by PROSAB are included in various parts of the book, representing invaluable and updated information applicable to warm climate regions. Volumes 1 to 5 were written by the two main authors. Volume 6 counted with the invaluable participation of Cleverson Vitorio Andreoli and Fernando Fernandes, who acted as editors, and of several specialists, who acted as chapter authors: Aderlene Inˆes de Lara, Deize Dias Lopes, Dione Mari Morita, Eduardo Sabino Pegorini, Hilton Fel´ıcio dos Santos, Marcelo Antonio Teixeira Pinto, Maur´ıcio Luduvice, Ricardo Franci Gon¸calves, Sandra M´arcia Ces´ario Pereira da Silva, Vanete Thomaz Soccol. Many colleagues, students and professionals contributed with useful suggestions, reviews and incentives for the Brazilian books that were the seed for this international version. It would be impossible to list all of them here, but our heartfelt appreciation is acknowledged. The authors would like to express their recognition for the support provided by the Department of Sanitary and Environmental Engineering at the Federal University of Minas Gerais, Brazil, at which the two authors work. The department provided institutional and financial support for this international version, which is in line with the university’s view of expanding and disseminating knowledge to society. Finally, the authors would like to show their appreciation to IWA Publishing, for their incentive and patience in following the development of this series throughout the years of hard work. Marcos von Sperling Carlos Augusto de Lemos Chernicharo December 2006

The author

Marcos von Sperling PhD in Environmental Engineering (Imperial College, Univ. London, UK). Associate professor at the Department of Sanitary and Environmental Engineering, Federal University of Minas Gerais, Brazil. Consultant to governmental and private companies in the field of water pollution control and wastewater treatment. [email protected]

xi

1 Introduction to water quality and water pollution

1.1 INTRODUCTION Water, because of its properties as a solvent and its capacity to transport particles, incorporates in itself various impurities that characterise the water quality. Water quality is a result of natural phenomena and the acts of human beings. Generally it can be said that water quality is a function of land use in the catchment area. This is due to the following factors:

•

•


C

Natural conditions: even with the catchment area preserved in its natural condition, the surface water quality is affected by run off and infiltration resulting from rainfall. The impact of these is dependent on the contact of the water with particles, substances and impurities in the soil. Therefore, the incorporation of suspended solids (e.g. soil particles) or dissolved solids (e.g. ions originating from the dissolution of rocks) occurs even when the catchment area is totally preserved in its natural condition (e.g. occupation of the land with woods and forests). In this case, the soil protection and composition have a great influence. Interference of human beings: the interference of man manifests itself either in a concentrated form, such as in the discharge of domestic or industrial wastewater, or in a diffused form, such as in the application of fertilisers or pesticides onto the soil. Both contribute to the introduction of

2007 IWA Publishing. Wastewater Characteristics, Treatment and Disposal by Marcos von Sperling. ISBN: 1 84339 161 9. Published by IWA Publishing, London, UK.

2

Wastewater characteristics, treatment and disposal compounds into the water, thus affecting its quality. Therefore, the form in which human beings use and occupy the land has a direct implication in the water quality.

Figure 1.1 presents an example of possible interactions between land use and the presence of factors that modify the water quality in rivers and lakes. Water quality control is associated with a global planning at the whole catchment area level, and not individually, for each impacting source.

Figure 1.1. Examples in a catchment area of the interrelation between land use and water quality impacting agents

Separate from the above concept of existing water quality, there is the concept of the desired water quality. The desired quality for a water is a function of its intended use. There are various possible intended uses for a particular water, which are listed in Section 1.2. In summary:

• •

Existing water quality: function of the land use in the catchment area Desired water quality: function of the intended uses for the water

Within the focus of this book, the study of water quality is essential, not only to characterise the consequences of a certain polluting activity, but also to allow the selection of processes and methods that will allow compliance with the desired water uses.

Introduction to water quality and water pollution

3

1.2 USES OF WATER The main water uses are:

• • • • • •

domestic supply industrial supply irrigation animal supply preservation of aquatic life recreation and leisure

• • • • •

breeding of aquatic species generation of electricity navigation landscape harmony dilution and transport of wastes

In general terms, only the first two uses (domestic supply and industrial supply) are frequently associated with a prior water treatment, in view of their more demanding quality requirements. There is a direct relation between water use and its required quality. In the above list, the most demanding use can be considered domestic water supply, which requires the satisfaction of various quality criteria. Conversely, the less demanding uses are simple dilution and transportation of wastes, which do not have any specific requirements in terms of quality. However, it must be remembered that multiple uses are usually assigned to water bodies, resulting in the necessity of satisfying diverse quality criteria. Such is the case, for example, of reservoirs constructed for water supply, electricity generation, recreation, irrigation and others. Besides the cycle of water on Earth (hydrological cycle), there are internal cycles, in which water remains in the liquid state, but has its characteristics modified as a result of its use. Figure 1.2 shows an example of typical routes of water use, composing partial cycles. In these cycles, the water quality is modified at each stage of its journey. The management of these internal cycles is an essential role in environmental engineering, and includes the planning, design, construction and control of the works necessary for the maintenance of the desired water quality as a function of its intended uses. Therefore, the engineer or scientist must know how to ask for and interpret the results of water quality samples in the various points of the cycle. This book focuses mainly on the aspect of wastewater treatment, and the impact of the discharge of wastewater to receiving bodies is covered in Chapter 3.

1.3 WATER QUALITY REQUIREMENTS Table 1.1 presents in a simplified way the association between the main quality requirements and the corresponding water uses. In cases of water bodies with multiple uses, the water quality must comply with the requirements of the various intended uses. The expression “free” in the table is different from “absolutely free”. Zero levels of many contaminants cannot be guaranteed and in most cases are not necessary. The acceptable concentrations are based on risk analysis, a tool that is used for deriving quality guidelines and standards.

4

Wastewater characteristics, treatment and disposal

•

Raw water. Initially, water is abstracted from the river, lake or water table, and has a certain quality. Treated water. After abstraction, water undergoes transformations during its treatment to be able to comply with its intended uses (e.g. public or industrial water supply). Raw wastewater. The water, after being used, undergoes new transformations in its quality and becomes a liquid waste. Treated wastewater. Aiming at removing its main pollutants, wastewater undergoes treatment before being discharged into the receiving body. Wastewater treatment is responsible for the new modification in the quality of the liquid. Stormwater. Rain water flows on the ground, incorporates some pollutants, and is collected at stormwater systems before being discharged into the receiving body. Receiving body. Stormwater and the effluent from the wastewater treatment plant reach the receiving body where water quality undergoes new modifications, as a result of dilution and self-purification mechanisms.

• • • • •

Figure 1.2. Routes of water use and disposal

Introduction to water quality and water pollution

5

Table 1.1. Association between water use and quality requirements General use Domestic supply

Specific use –

– – – –

Industrial supply

– –

Free from chemical substances harmful to health Free from organisms harmful to health Aesthetically pleasant (low turbidity, colour, taste and odour; absence of macro-organisms)

–

Variable with the product

Water that does not enter into contact with the product (e.g. refrigeration units, boilers)

– –

Low hardness Low aggressiveness

Horticulture, products ingested raw or with skin

–

Free from chemical substances harmful to health Free from organisms harmful to health Non-excessive salinity

Other plantations

–

–

– –

Water incorporated into the product (e.g. food, drinks, medicines)

Water that enters into contact with the product

Irrigation

Animal water supply

–

Required quality Free from chemical substances harmful to health Free from organisms harmful to health Low aggressiveness and hardness Aesthetically pleasant (low turbidity, colour, taste and odour; absence of macro-organisms)

– –

–

Free from chemical substances harmful to the soil and plantations Non-excessive salinity Free from chemical substances harmful to animals health Free from organisms harmful to animals health

Preservation of aquatic life

–

–

Variable with the environmental requirements of the aquatic species to be preserved

Aquaculture

Animal breeding

–

Free from chemical substances harmful to animals, workers and consumers health Free from organisms harmful to animals, workers and consumers health Availability of nutrients

– – Vegetable growing

– –

Free from chemical substances toxic to vegetables and consumers Availability of nutrients (continued)

6

Wastewater characteristics, treatment and disposal

Table 1.1. (continued) General use Recreation and leisure

Specific use Primary contact (direct contact with the liquid medium – bathing; e.g.: swimming, water-skiing, surfing)

– – –

Required quality Free from chemical substances harmful to health Free from organisms harmful to health Low levels of suspended solids and oils and grease

Secondary contact (without direct contact with the liquid medium; e.g.: leisure navigation, fishing, contemplative viewing)

–

Pleasant appearance

Energy generation

Hydroelectric power plants

–

Low aggressiveness

Nuclear or thermoelectric power plants (e.g. cooling towers)

–

Low hardness

Transport

–

–

Low presence of course material that could be dangerous to vessels

Waste dilution and transportation

–

–

1.4 WATER POLLUTION Water pollution is the addition of substances or energy forms that directly or indirectly alter the nature of the water body in such a manner that negatively affects its legitimate uses. This definition is essentially practical and, as a consequence, potentially controversial, because of the fact that it associates pollution with negative alterations and with water body uses, concepts that are attributed by human beings. However, this practical view is important, principally when analysing the control measures for pollution reduction Table 1.2 lists the main pollutants and their source, together with the most representative effects. Chapter 2 covers in detail the main parameters, which characterise the quality of a wastewater (second column in the table). For domestic sewage, which is the main focus of this book, the main pollutants are: suspended solids, biodegradable organic matter, nutrients and pathogenic organisms. Their impact in the water body is analysed in detail in Chapter 3. The solution to most of these problems, especially biodegradable organic matter and pathogens, has been reached in many developed regions, which are now concentrated on the removal of nutrients and micro-pollutants, together with substantial attention to the pollution caused by storm-water drainage. In developing

7

Total dissolved solids Conductivity

Inorganic dissolved solids

xxx: high

Specific elements (As, Cd, Cr, Cu, Hg, Ni, Pb, Zn, etc.)

Metals

xx: medium

Pesticides Some detergents Others

Non-biodegradable organic matter

x: small

Coliforms

Nitrogen Phosphorus

Nutrients

Pathogens

Biochemical oxygen demand

Biodegradable organic matter

Pollutant Suspended solids

Main representative parameters Total suspended solids

X

←→

X

XX

X

X

X

empty: usually not important

X

←→

←→

XX

XX

←→

←→

XX

←→

Agricultural and pasture X

Stormwater Urban XX

Source

Industrial ←→

←→: variable

XX

X

X

XXX

XXX

XXX

Domestic XXX

Wastewater

Table 1.2. Main pollutants, their source and effects

• •

• • •

• • • • • • • • • • • • • • • • • • •

Toxicity (various) Foam (detergents) Reduction of oxygen transfer (detergents) Non-biodegradability Bad odours (e.g.: phenols) Toxicity Inhibition of biological sewage treatment Problems in agriculture use of sludge Contamination of groundwater Excessive salinity – harm to plantations (irrigation) Toxicity to plants (some ions) Problems with soil permeability (sodium)

Possible effect of the pollutant Aesthetic problems Sludge deposits Pollutants adsorption Protection of pathogens Oxygen consumption Death of fish Septic conditions Excessive algae growth Toxicity to fish (ammonia) Illnesses in new-born infants (nitrate) Pollution of groundwater Water-borne diseases

8

Wastewater characteristics, treatment and disposal

regions, the basic pollution problems still need to be dealt with, and the whole array of pollutants needs to be tackled. However, because of scarcity of financial resources in these regions, priorities need to be set (as they have been, in the past, and continue to be, in the developed regions), and the gross pollution by organic matter and contamination by pathogens are likely to deserve higher attention. Naturally, each region has its own specificities, and these need to be taken into account when setting up priorities. Also from the table, it is seen that it is very difficult to generalise industrial wastewater, because of its variability from process to process and from industry to industry. In the table, it is also seen that there are two ways in which the pollutant could reach the receiving body (see Figure 1.3):

• •

point-source pollution diffuse pollution

Figure 1.3. Point-source and diffuse pollution

In point-source pollution, the pollutants reach the water body in points concentrated in the space. Usually the discharge of domestic and industrial wastewater generates point-source pollution, since the discharges are through outfalls. In diffuse pollution, the pollutants enter the water body distributed at various locations along its length. This is the typical case of storm water drainage, either in rural areas (no pipelines) or in urban areas (storm water collection system, with multiple discharges into the water body). The focus of this book is the control of point-source pollution by means of wastewater treatment. In the developing regions, there is practically everything still to be done in terms of the control of point-source pollution originating from cities and industries.

2 Wastewater characteristics

2.1 WASTEWATER FLOWRATES 2.1.1 Introduction Wastewater sewerage (collection, treatment and disposal) is accomplished by the following main alternatives (Figure 2.1):

•

•

Off-site sewerage • Separate sewerage system • Combined sewerage system On-site sewerage

In various countries a separate sewerage system is adopted, which separates storm water from sewage, both being transported by independent pipeline systems. In this case, in principle, storm water does not contribute to the wastewater treatment plant (WWTP). In other countries, however, a combined (unitary) sewerage system is adopted, which directs sewage and storm water together into the same system (see Figure 2.1). In this case, the pipelines have a larger diameter, to transport not only the sewage flow, but mainly rainwater, and the design of the WWTP has to take into consideration the corresponding fraction of rainwater that is allowed to enter the treatment works. In countries with a warm climate, during the dry season, sewage flows slowly in these large diameter pipes, leading to long detention times which allow decomposition and generation of malodours. In this book, focus is given to the separate sewerage system, analysing only the three components listed above. However, the principles for the design of a combined sewerage system, based on dry-weather flow, are the same.
C

2007 IWA Publishing. Wastewater Characteristics, Treatment and Disposal by Marcos von Sperling. ISBN: 1 84339 161 9. Published by IWA Publishing, London, UK.

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Wastewater characteristics, treatment and disposal

(a) Sewerage systems: on-site and off-site

(b) Off-site sewerage systems: separate and combined

Figure 2.1. Types of sewerage system

Similarly, the book concentrates on off-site collection systems (with a waterborne sewerage collection and transportation network) and does not cover the on-site systems (e.g. latrines and septic tanks). These are of great importance and in many cases the best alternative in various regions, being more applicable in locations with a low population density, like rural areas (even though they are also applied in various densely occupied locations, but frequently presenting problems of infiltration in the soil and the resulting contamination of the water table). Urban wastewater that flows in an off-site sewerage system and contributes to a WWTP is originated from the following three main sources:

• • •

Domestic sewage (including residences, institutions and commerce) Infiltration Industrial effluents (various origins and types of industry)

For the characterisation of both quantity and quality of the influent to the WWTP, it is necessary to separately analyse each of the three items.

2.1.2 Domestic wastewater flow 2.1.2.1 Preliminaries The concept of domestic flow encompasses the sewage originating from homes, as well as commercial activities and institutions that are normally components of the

Wastewater characteristics

11

locality. More expressive values originating from significant point sources must be computed separately and added to the global value. Normally domestic sewage flow is calculated based on the water consumption in the respective locality. The water consumption is usually calculated as a function of the design population and of a value attributed for the average daily per capita water consumption. It is important to observe that for the design and operation of the sewage treatment works it is not sufficient to consider only the average flow. It is also necessary to quantify the minimum and maximum flowrates, because of hydraulic and process reasons. This Section describes the population-forecast studies, the estimates of water consumption and the production of wastewater, together with the variations in flow (minimum and maximum flow).

2.1.2.2 Population forecast The population that contributes to the treatment plant is that situated inside the design area served by the sewerage system. However, the design population is only a certain fraction of the total population in this area, because maybe not all the population is connected to the sewerage system. This ratio (population served/total population) is called the coverage index. This index can be determined (current conditions) or estimated (future conditions), such as to allow the calculation of the design flow. In the final years of the planning horizon, it is expected that the coverage will be close to 100%, reflecting the improvement and expansion in the collection network. The coverage index is a function of the following aspects:

•

•

•

Physical, geographical or topographical conditions of the locality. It is not always possible to serve all households with the sewerage system. Those not served must adopt other solutions besides the off-site water-borne sewerage system. Adhesion index. This is the ratio between the population actually connected to the system and the population potentially served by the sewerage system in the streets (not all households are connected to the available system, that is to say, not all adhere to the sewerage system). In some communities, it is compulsory to connect to the collection system, in case it passes in front of the house; in other communities, this is optional. Implementation stages of the sewerage system. In the initial operating years of the WWTP, maybe not all of the designed collection and transport system has been actually installed, and this affects the initial flow.

For the design of a sewage treatment works it is necessary to know the final population (population at the end of the planning horizon – see Chapter 6 for the concept of planning horizon) as well as the initial population and its evolution with time, in order to allow the definition of implementation stages.

Description

Decreasing growth rate

Assumption that, as the town grows, the growth rate becomes lower. The population tends asymptotically to the saturation value. The coefficients can be also estimated by non-linear regression.

Population growth follows a constant rate. Method used for short-term forecasts. Curve fitting can also be done through regression analysis. Geometric Population growth is a function growth of the existing population at every instant. Used for short-term forecasts. Curve fitting can also be done through regression analysis. Multiplicative Fitting of population growth by regression linear regression (logarithmic transformation of the equation) or non-linear regression.

Linear growth

Method

Curve shape

Table 2.1. Population forecast. Methods based on mathematical formulas

Pt = Po + r.(t − to )s

–

Pt = P
0 + (Ps − Po )  × 1 − e−Kd .(t−t0 )

Pt = Po .eKg .(t−t0 ) or Pt = Po .(1 + i)(t−t0 )

dP = Kg .P dt

dP = Kd .(Ps − P) dt

Pt = Po + Ka .(t − to )

Forecast formula

dP = Ka dt

Growth rate

P2 − P0 t2 − t 0

2.Po .P1 .P2 − P1 2 .(Po + P2 ) Po .P2 − P1 2 −ln[(Ps − P2 )/(Ps − Po )] Kd = t2 − t o Ps =

r, s – regression analysis

Kg =

lnP2 − lnP0 t2 − t 0 or i = eKg − 1

Ka =

Coefficients (if regression analysis is not used)

The population growth follows an S-shaped curve. The population tends asymptotically to a saturation value. The coefficients can be also estimated by non-linear regression. Required conditions: Po < P1 < P2 and P0 .P2 < P1 2 . The point of inflexion in the curve occurs at time t = [to − ln(c)/K1 ] and with Pt = Ps /2. K1 =

  1 Po .(Ps − P1 ) .ln t2 − t1 P1 .(Ps − Po )

c = (Ps − Po )/Po

  2.Po .P1 .P2 − P1 2 .(Po + P2 ) (Ps − P) Ps dP Ps = = K1 .P. Pt = Po .P2 − P1 2 dt Ps 1 + c.eK1 .(t−to )

• Pt = population estimated for year t (inhabitants); Ps = saturation population (inhabitants) • Ka , Kg , Kd , Kl , i, c, r, s = coefficients (obtaining coefficients by regression analysis is preferable as all of the existing data series can be used, and not only Po , P1 e P2 )

employed (inhabitants)

• dP/dt = population growth rate as a function of time • Po , P1 , P2 = population in the years to , t1 , t2 . The formulas for the decreasing and logistic growth rates require equally-spaced values in time if regression analysis is not

Source: partly adapted from Qasim (1985)

Logistic growth

14

Wastewater characteristics, treatment and disposal

The main methods or models used for population forecasts are (Fair et al, 1973; CETESB, 1978; Barnes et al, 1981; Qasim, 1985; Metcalf & Eddy, 1991):

• • • • • • • •

linear (arithmetic) growth geometric growth multiplicative regression decreasing growth rate logistic growth graphical comparison between similar communities method of ratio and correlation prediction based on employment forecast or other utilities forecast

Tables 2.1 and 2.2 list the main characteristics of the various methods. All of the methods presented in Table 2.1 can also be solved through statistical regression analysis (linear or non-linear). Such methods are found in many commercially available computer programs. Whenever possible it is always best to adopt a regression analysis that allows the incorporation of a largest historical data series instead of two or three 3 points, such as the algebraic methods presented in Table 2.1. The results of the population forecast must be coherent with the population density in the area under analysis. The population density data are also useful in the computation of flows and loads resulting from a certain area or basin within the town. Typical population density values are presented in Table 2.3. Table 2.4 presents typical saturation population densities, in highly occupied metropolitan areas. When making population forecasts, the following points must be taken into consideration:

•

•

•

The population studies are normally very complex. All the variables (unfortunately not always quantifiable) that could interact in the specific locality under study must be analysed. Unexpected events can still occur, which can completely change the predicted trajectory of the population growth. This emphasises the need to establish a realistic value for the planning horizon and for the implementation stages of the WWTP. The mathematical sophistication associated with the determination of the coefficients of some forecast equations loses its meaning if it is not based on parallel information, often non-quantifiable, such as social, economical, geographical and historical aspects. The common sense of the analyst is very important in the choice of the forecast and in the interpretation of the results. Even though the choice of method is based on the best fit with census data, the extrapolation of the curve requires perception and caution.

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15

Table 2.2. Population forecast based on indirect quantification methods Method Graphical comparison

Ratio and correlation

Forecast of employment and utility services

Description The method involves the graphical fitting of the past population under study. The population data of other similar but larger towns are plotted in a manner that the curves coincide at the current value of the population of the town under study. These curves are used as references for the forecast of the town under study. It is assumed that the town population follows the same trend of the region (physical or political region) in which it is inserted. Based on the census records, the ratio “town population / region population” is calculated and projected for future years. The town population is obtained from the population forecast for the region (made at a planning level by another body) and the calculated ratio. The population is estimated using a job prediction (made by another body). Based on the past population data and people employed, the “job/population” ratio is calculated and projected for future years. The town population is calculated from the forecast of the number of jobs in the town. The procedure is similar to the ratio method. The same methodology can be adopted from the forecast of utility services, such as electricity, water, telephone, etc. The service utility companies normally undertake studies of forecast and expansion of their services with relative reliability.

Note: The forecast of the ratios can be done based on regression analysis Source: Qasim (1985)

Table 2.3. Typical population densities as a function of land use Population density Land use Residential areas • single-family dwellings, large lots • single-family dwellings, small lots • multiple-family dwellings, small lots Apartments Commercial areas Industrial areas Total (excluding parks and other large-scale equipment)

(inhab/ha)

(inhab/km2 )

12–36 36–90 90–250 250–2,500 36–75 12–36 25–125

1,200–3,600 3,600–9,000 9,000–25,000 25,000–250,000 3,600–7,500 1,200–3,600 2,500–12,500

Source: adapted from Fair et al (1973) and Qasim (1985) (rounded up values)

16

Wastewater characteristics, treatment and disposal

Table 2.4. Population densities and average street length per hectare, under saturation conditions, in highly occupied metropolitan areas Land use High standard residential areas, with standard lots of 800 m2 Intermediate standard residential areas, with standard lots of 450 m2 Popular residential areas, with standard lots of 250 m2 Centrally-located mixed residential–commercial areas, with predominance of 3–4 storey buildings Centrally-located residential areas, with predominance of 10–12 storey buildings Mixed residential–commercial–industrial urban areas, with predominance of commerce and small industries Centrally-located residential areas, with predominance of office buildings

Saturation population density (inhab/ha) 100

Average street length (m/ha) 150

120

180

150

200

300

150

450

150

600

150

1000

200

Average data from S˜ao Paulo Metropolitan Area, Brazil Source: Alem Sobrinho and Tsutiya (1999)

Example 2.1 Based on the following census records, undertake the population forecast using the methods based on mathematical formulas (Table 2.1). Data: Year

Population (inhabitants)

1980 1990 2000

10,585 23,150 40,000

Solution: a)

Nomenclature of the years and populations

According to Table 2.1, there is the following nomenclature: t0 = 1980 P0 = 10,585 inhab t1 = 1990 P1 = 23,150 inhab t2 = 2000 P2 = 40,000 inhab b)

Linear (arithmetic) growth Ka =

P2 − Po 40000 − 10585 = 1470.8 = t2 − to 2000 − 1980

Pt = Po + Ka .(t − to ) = 10585 + 1470.8 × (t − 1980)

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17

Example 2.1 (Continued) For example, to calculate the population in the year 2005, t is substituted for 2005 in the above equation. For the year 2010, t = 2010, and so on. c)

Geometric growth Kg =

lnP2 − lnPo ln 40000 − ln 10585 = 0.0665 = t2 − t o 2000 − 1980

Pt = P0 .eKg .(t−t0 ) = 10585.e0.0665×(t−1980) d)

Decreasing growth rate

Ps =

2.Po .P1 .P2 − P1 2 .(Po + P2 ) Po .P2 − P1 2

2 × 10585 × 23150 × 40000 − 231502 × (10585 + 40000) 10585 × 40000 − 231502 = 66709 =

The saturation population is, therefore, 66,709 inhabitants. Kd =

−ln[(Ps − P2 )/(Ps − Po )] t2 − to

−ln[66709 − 40000)/(66709 − 10585)] = 0.0371 2000 − 1980 
Pt = PO + (Ps − Po ). 1 − e−Kd .(t−to )   = 10585 + (66709 − 10585) × 1 − e−0,0371×(t−1980) =

e)

Logistic growth

Ps = = = c= K1 = =

2.Po .P1 .P2 − P1 2 .(Po + P2 ) Po .P2 − P1 2 2 × 10585 × 23150 × 40000 − 231502 × (10585 + 40000) 10585 × 40000 − 231502 66709 (Ps − Po ) (66709 − 10585) = 5.3022 = Po 10585   Po .(Ps − P1 ) 1 .ln t2 − t 1 P1 .(Ps − Po )   10585 × (66709 − 23150) 1 .ln = −0,1036 2000 − 1990 23150 × (66709 − 10585)

18

Wastewater characteristics, treatment and disposal Example 2.1 (Continued)

Pt =

Ps 66709 = K .(t−t ) 1 0 1 + c.e 1 + 5.3022.e−0.1036×(t−1980)

The inflexion in the S-shaped curve occurs at the following year and population: Inflexion time = to −

ln(c) ln(5.3022) = 1980 − = 1996 K1 −0.1036

Population at inflexion =

66709 Ps = = 33354 inhab 2 2

Before inflexion (year 1996), population growth presented an increasing rate and, after it, a decreasing rate. f)

Results in table and graphic form

Population forecast Actual population Decreasing Nomenclature Year (census) Linear Geometric rate Logistic P0 P1 P2 – – – –

1980 1990 2000 2005 2010 2015 2020

10585 23150 40000 – – – –

10585 25293 40000 47354 54708 62061 69415

10585 20577 40000 55770 77758 108414 151157

10585 27992 40000 44525 48284 51405 53998

10585 23150 40000 47725 53930 58457 61534

POPULATION FORECAST 80000

POPULATION (inhab)

70000 60000 50000 40000

CENSUS Logistic

30000

Arithmetic

20000

Geometric Decreasing

10000

Saturation

0 1980

1985

1990

1995

2000

2005

2010

2015

YEAR

Population forecast. Census and estimated data

2020

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19

Example 2.1 (Continued) From the graph and table, the following points specific for this data group can be seen:

•

• • • •

•

•

The census data (population of the years 1980 to 2000) present an increasing growth rate trend. Visually, it is seen that the decreasing rate model does not fit well. The geometric method leads to very high future estimates (that can turn out to be true or not, but that are far away from the other forecasts). The logistic and decreasing rate methods tend to the saturation population. (66,709 inhabitants, indicated on the graph) In all methods, the calculated population values for the years P0 and P2 are equal to the measured values. The population forecast as such is only from year 2000. The years with census data are plotted to permit the visual interpretation of the fit of the curves to the measured data (1980, 1990, 2000). The best-fit curve may be chosen from statistical criteria, which give an indication of the prediction error (usually based on the sum of the squared errors), where error or residual is the difference between the estimated and the observed data. Spreadsheets may be used, to find the value of the coefficients that lead to the minimum sum of the squared errors (e.g. solver tool in Excel® ).

2.1.2.3 Average water consumption As mentioned, the domestic flow is a function of the water consumption. Typical values of per capita water consumption for populations provided with household water connections are presented in Table 2.5. These values can vary from locality to locality. Table 2.6 presents various factors that influence water consumption. The data listed in Table 2.5 are simply typical average values, being naturally subjected to all the variability resulting from the factors listed in Table 2.6. Table 2.5. Typical ranges of per capita water consumption Community size Rural settlement Village Small town Average town Large city

Population range (inhabitants) 250,000

Per capita water consumption (L/inhab.d) 90–140 100–160 110–180 120–220 150–300

Note: in places with severe water shortages, these values may be smaller Source: Adapted from CETESB (1977; 1978), Barnes et al (1981), Dahlhaus & Damrath (1982), Hosang & Bischof (1984)

20

Wastewater characteristics, treatment and disposal

Table 2.6. Factors that influence water consumption Influencing factor Water availability

•

•

Climate

•

•

Community size

•

•

Economic level of the community

•

•

Level of industrialisation

•

• • •

Metering of household consumption Water cost Water pressure

• • •

•

System losses

•

•

Comment In locations of water shortage consumption tends to be less Warmer climates induce a greater water consumption Larger cities generally present a larger per capita water consumption (to account for strong commercial and institutional activities) A higher economic level is associated with a higher water consumption Industrialised locations present a higher consumption Metering inhibits greater consumption A higher cost reduces consumption High pressure in the distribution system induces greater use and wastage Losses in the water distribution network imply the necessity of a greater water production

WATER FLOW vs NUMBER OF MINIMUM SALARIES y=x/((0.021)+(0.003)*x)

WATER FLOW (L/inhab.d)

300 250 200 150 100 50 0

0

4

8

12

16

20

NUMBER OF MINIMUM SALARIES

Figure 2.2. Per capita water consumption as a function of family salary in Belo Horizonte, Brazil

Campos and von Sperling (1996) observed, for predominantly residential sewage originating from nine sub-catchment areas in Belo Horizonte, Brazil, a strong relationship between per capita water consumption and average monthly family income (in number of minimum salaries) (Figure 2.2). Naturally the data are site specific and require caution in their extrapolation to other conditions. Water consumption data from 45 municipalities in the State of Minas Gerais, Brazil (von Sperling et al, 2002), were investigated by the author. The State of Minas Gerais has many features in common with Brazil, as a whole, and many

Wastewater characteristics

21

Per capita water consumption (L/inhb.d)

Per capita municipal income x per capita water consumption 300 250 200 150 100 50 0 10

100

1000

Per capita income (US$/inhab.year)

Figure 2.3. Relationship between per capita water consumption and per capita income. Data from the state of Minas Gerais, Brazil (von Sperling et al, 2002) (US$1.00 = R$2.50)

other developing countries, because it presents regions with high and low economic level, rainfall and temperature. The range of variation in the data was: per capita water consumption: 84 to 248 l/inhab.d; urban population: 4,000–2,300,000 inhabitants; average per capita income: US$8–1600 per inhabitant per year; mean yearly temperature: 20–26◦ C; mean yearly rainfall: 300–1750 mm/year. Figure 2.3 presents the relation of the per capita water consumption with per capita income, which was the clearer one. The analysis should be done only in terms of trends and average values, since the correlation coefficient was not high, as a result of a substantial scatter in the data. Figure 2.4 presents the ranges of variation of the per capita water consumption as a function of the category of the per capita income and rainfall of the 45 municipalities (separation between low and high income: US$110/inhab.year, corresponding to the median of values; separation of high and low rainfall: 1350 mm/year, corresponding to the average value of the State of Minas Gerais). Naturally these values are region specific, but it is believed that a certain extrapolation of trends and ranges can be done, but always judiciously. Table 2.7 shows ranges of per capita water consumption as a function of income and rainfall, based on the 25 and 75 percentiles presented in Figure 2.4. Tables 2.8 and 2.9 show the ranges of average water consumption values for various commercial establishments and institutions. This information, which should only be used in the absence of more specific data, is particularly useful in the design of sewage treatment works for small areas, in which the contribution of individual important establishments could have an importance in the general flow calculations.

2.1.2.4 Average sewage flow In general, the production of sewage corresponds approximately to the water consumption. However, the fraction of the sewage that enters the sewerage system can

22

Wastewater characteristics, treatment and disposal Table 2.7. Ranges of water consumption values, based on 45 municipalities in the State of Minas Gerais, Brazil

Income Low High

Ranges of per capita water consumption (L/inhab.d) Low rainfall High rainfall 120–165 130–190 140–180 150–200

Notes:

• Ranges based on 25 and 75 percentile values from Fig. 2.4 • In larger towns (greater than 200,000 inhabitants), the per capita water consumption was on average approximately 10% higher than in smaller towns • The ranges present usual values, and it is frequent to observe values outside them

PER CAPITA WATER CONSUMPTION AS A FUNCTION OF INCOME CLASSES AND RAINFALL

PER CAPITA CONSUMPTION (L/inhab.d)

250

200

150

100

Min-Max 50 LOW INCOMEHIGH INCOME

LOW INCOMEHIGH INCOME

25–75%

RAINFALL: LOW

RAINFALL: HIGH

Median

Figure 2.4. Box-and-whisker plot of the per capita water consumption values as a function of categories for per capita income and mean yearly rainfall (45 municipalities in the State of Minas Gerais, Brazil)

be different, due to the fact that part of the water consumed could be incorporated into the storm water system or infiltrate (e.g. watering of gardens and parks). Other influencing factors in a separate sewerage system are: (a) clandestine sewage connections to the storm water system, (b) clandestine connections of storm water into the separate sewerage system and (c) infiltration. The last point is covered separately in Section 2.1.3. The fraction of the supplied water that enters the sewerage system in the form of sewage is called Return Coefficient (R = sewage flow/water flow). Typical values vary between 60% and 100%, and a value of 80% (R = 0.8) is usually adopted.

Wastewater characteristics

23

Table 2.8. Typical water consumption in some commercial establishments Establishment Airport Accommodation (lodging house) Public toilet Bar Cinema/theatre Office Hotel Industry (sanitary sewage only) Snack bar Laundry – commercial Laundry – automatic Shop Department store Petrol station Restaurant Shopping centre

Unit Passenger Resident User Customer Seat Employee Guest Employee Employee Customer Machine Machine Toilet Employee Toilet Employee m2 of area Vehicle attended Meal Employee m2 of area

Flow range (L/unit.d) 8–15 80–150 10–25 5–15 2–10 30–70 100–200 30–50 50–80 4–20 2,000–4,000 1,500–2,500 1,000–2,000 30–50 1,600–2,400 30–50 5–12 25–50 15–30 30–50 4–10

Source: EPA (1977), Hosang and Bischof (1984), Tchobanoglous and Schroeder (1985), Qasim (1985), Metcalf & Eddy (1991), NBR-7229/93

Table 2.9. Typical water consumption in some institutional establishments Establishment Rest home School – with cafeteria, gymnasium, showers – with cafeteria only – without cafeteria and gymnasium Hospital Prison

Unit Resident Employee Student Student Student

Flow range (L/unit.d) 200–450 20–60 50–100 40–80 20–60

Bed Employee Inmate Employee

300–1000 20–60 200–500 20–60

Source: EPA (1977), Hosang and Bischof (1984), Tchobanoglous and Schroeder (1985), Qasim (1985), Metcalf & Eddy (1991)

The average domestic sewage flow calculation is given by: Qdav =

Pop.Lpcd .R 1000

(m3/d)

(2.2)

Qdav =

Pop.Lpcd .R 86400

(L/s)

(2.3)

24

Wastewater characteristics, treatment and disposal

where: Qdav = average domestic sewage flow (m3 /d or L/s) Lpcd = per capita water consumption (L/inhab.d) R = sewage flow/water flow return coefficient It is important to notice that the water flow to be considered is the flow actually consumed, and not the flow produced by the water treatment works. The water flow produced is higher than that consumed due to unaccounted water losses in the distribution system, which can vary typically from 20 to 50%. Thus in a locality where the loss is 30%, for each 100 m3 of water produced, 30 m3 are unaccounted for and only 70 m3 are consumed. Of this 70 m3 , around 80% (56 m3 /d) return in the form of sewage to the sewerage system.

2.1.2.5 Flow variations. Maximum and minimum flows Water consumption and wastewater generation in a locality vary throughout the day (hourly variations), during the week (daily variations) and throughout the year (seasonal variations). Figure 2.5 presents typical hourly influent flowrate variations in a WWTP. Two main peaks can be observed: a peak at the beginning of the morning (more pronounced) and a peak at the beginning of the evening (more distributed). The average daily flow corresponds to the line that separates equal areas, below and above the line.

FLOW Qmax

Qav Qmin

0

6

12

18

24

hours of the day

Figure 2.5. Typical hourly flow variations in the influent to a sewage treatment works

The following coefficients are frequently used to allow the estimation of minimum and maximum water flows:

• • •

K1 = 1.2 (peak coefficient for the day with the highest water consumption) K2 = 1.5 (peak coefficient for the hour with the highest water consumption) K3 = 0.5 (reduction coefficient for the hour with the lowest water consumption)

Wastewater characteristics

25

Table 2.10. Coefficient of hourly variation of sewage flow Qmax /Qav 1 + (14/(4+P0.5 )) 5P−0.16

Qmin /Qav – 0.2P0.16

Author Harmon Gifft

Reference Qasim (1985) Fair et al (1973)

Notes: P = population, in thousands Gifft’s formula is indicated for P < 200 (population < 200,000 inhabitants)

Thus, the maximum and minimum water flows can be given by the formulas: Qmax = Qav . K1 . K2 = 1.8 Qav

(2.4)

Qmin = Qav . K3 = 0.5 Qav

(2.5)

If it is possible to carry out flow measurements, to establish the real flow variations, the actual data should be used in the design. The coefficients K1 , K2 and K3 are generalised, thus probably not allowing the accurate reproduction of the flow variations in the locality under analysis. Over- or underestimated values affect directly the technical and economical performance of the sewage works design. When considering hourly variations of wastewater flow, it should be taken into consideration that the fluctuations are absorbed and reduced in amplitude along the sewerage system. It is easy to understand that the larger the network (or the population), the lower are the chances of peak flows to overlap simultaneously in the works entrance. Thus the residence time in the sewerage system has a large influence on the absorption of the peak flows. Based on this concept, some authors have developed formulas for correlating the coefficients of variation with population, or with average flow (Table 2.10). As an illustration, the following table presents the calculated coefficients for different populations.

Qmax /Qav Population 1,000 10,000 100,000 1,000,000

Harmon 3.8 3.0 2.0 1.4

Gifft 5.0 3.4 2.3 -

Qmin /Qav Gifft 0.20 0.14 0.09 -

It can be observed that even the product of the coefficients K1 and K2 utilised for water supply, and frequently adopted as 1.8, could induce an underestimated ratio Qmax /Qav for a wide population range.

26

Wastewater characteristics, treatment and disposal

Table 2.11. Approximate values of infiltration rates in sewerage systems Pipe diameter

Type of joint

Groundwater level Below the pipes

Elastic

L/s.km 0.05 0.10

m3 /d.km 4 9

Low High

0.15 0.30

13 26

Below the pipes

Low High

0.05 0.50

4 43

Low High –

0.50 1.00 1.00

43 86 86

Non-elastic Above the pipes –

Infiltration coefficient

Above the pipes < 400 mm

> 400 mm

Soil permeability Low High

–

Source: Crespo (1997)

2.1.3 Infiltration flow Infiltration in a sewerage system occurs through defective pipes, connections, joints or manholes. The quantity of infiltrated water depends on various factors, such as the extension of the collection network, pipeline diameters, drainage area, soil type, water table depth, topography and population density (number of connections per unit area) (Metcalf & Eddy, 1991). When no specific local data are available, infiltration rate is normally expressed in terms of flow per extension of the sewerage system or per area served. The values presented in Table 2.11 can be used as a first estimate, when no specific local data are available (Crespo, 1997). Metcalf & Eddy (1991) present the infiltration coefficient as a function of the pipe diameter: 0.01 to 1.0 m3 /d.km per mm. For instance, for a pipe diameter of 200 mm, the infiltration rate will range between 2 to 200 m3 /d.km. The length of the network may be measured in the locality by using the map of the location of the sewerage system. In the absence of these data (for instance, for future populations), in preliminary studies of smaller localities, where the population density is usually less, values around 2.5 to 3.5 m of network per inhabitant may be adopted. In medium-size cities this value could be reduced to around 2.0 to 3.0 m/inhab and in densely populated regions, even smaller values may be reached (1.0 to 2.0 m/inhab or even lower). Figure 2.6, based on the 45 municipalities described in Section 2.1.2, presents the ranges of variation of per capita length of sewerage network for two population categories. Based on the infiltration values per unit length and the per capita sewerage network length, per capita infiltration values may be estimated to range between 8 to 150 L/inhab.d, excluding the extreme values. In areal terms, based on typical population densities (25 to 125 inhab/ha), infiltration rates between 0.2 and 20 m3 /d per ha of drainage area (20 to 2000 m3 /d.km2 ) are obtained. These ranges are very wide, and the designer should analyse carefully the prevailing conditions in

Wastewater characteristics

27

PER CAPITA LENGTH OF THE SEWERAGE SYSTEM AS A FUNCTION OF THE POPULATION 5.0

PER CAPITA PIPE LENGTH (m/inhab)

4.5 4.0 3.5 3.0 2.5 2.0 1.5

Min–Max 25%–75%

1.0 POP 200,000 inhabitants

Median

Figure 2.6. Box-and-whisker plot of the per capita length of sewerage network, as a function of two categories of population size (45 municipalities in Minas Gerais, Brazil)

the sewerage network in order to obtain narrower ranges, which could best represent the specific conditions in the community under analysis. The utilisation of good materials and construction procedures helps in reducing the infiltration rates. In the calculation of the total influent flow to a WWTP, average infiltration values may be used for the computation of average and maximum influent flowrates. For minimum flow conditions, infiltration can be excluded, as a safety measure (in the case of minimum flow, the safety in a design is in the direction of establishing the lowest flow).

2.1.4 Industrial wastewater flow Industrial wastewater flow is a function of the type and size of the industry, manufacturing process, level of recycling, existence of pre-treatment, etc. Even in the case of two industries that manufacture essentially the same product, the wastewater flows can diverge substantially. If there are large industries contributing to the public sewerage system and subsequently to a WWTP, the adequate evaluation of their respective flows is of great importance. Industrial wastewater has a great influence in the planning and operation of a WWTP. Specific data must be obtained for each significant industry, through industrial surveys, thus allowing the supply of data of interest for the project. With relation to the water consumption and the generation of wastewater, the following information at least must be obtained for the main industries:

•

Water consumption • Total volume consumed (per day or month) • Volume consumed in the various stages of the process

28

Wastewater characteristics, treatment and disposal

•

• Internal recirculations • Water origin (public supply, wells, etc.) • Internal systems of water treatment Wastewater production • Total flow • Number of discharge points (with the corresponding industrial process associated with each point) • Discharge pattern (continuous or intermittent; duration and frequency) in each discharge point • Discharge destination (sewerage system, watercourse) • Occasional mixing of wastewater with domestic sewage and storm water

Additionally, whenever possible, effluent flow measurements must be carried out throughout the working day, to record the discharge pattern and variations. In the event of having no specific information available for the industry, Table 2.12 can be used as a starting point to allow estimation of the probable effluent flow range. These values are presented in terms of water consumption per unit of product manufactured. For simplicity it can be assumed that sewage flow is equal to water consumption. It can be seen from the table that there is a great variety of consumption values for the same type of industry. If there are no specific data available for the industry in question, specific literature references relative to the industrial process in focus must be consulted. The table presented only gives a starting point for more superficial or general studies. The daily discharge pattern for industrial wastewater does not follow the domestic flow variations, changing substantially from industry to industry. Industrial flow peaks do not necessarily coincide with the domestic peaks, that is to say, the total maximum flow (domestic + industrial) is normally less than the simple sum of the maximum flows.

2.2 WASTEWATER COMPOSITION 2.2.1 Quality parameters Domestic sewage contains approximately 99.9% water. The remaining part includes organic and inorganic, suspended and dissolved solids, together with microorganisms. It is because of this 0.1% that water pollution takes place and the wastewater needs to be treated. The composition of the wastewater is a function of the uses to which the water was submitted. These uses, and the form with which they were exercised, vary with climate, social and economic situation and population habits. In the design of a WWTP, there is normally no interest in determining the various compounds that make up wastewater. This is due, not only to the difficulty in

Table 2.12. Specific average flows from some industries

Type Food

Activity Canned fruit and vegetables Sweets Sugar cane Slaughter houses Dairy (milk) Dairy (cheese or butter) Margarine Brewery Bakery Soft drinks Textiles Cotton Wool Rayon Nylon Polyester Wool washing Dyeing Leather / Tannery tanneries Shoe Pulp and Pulp fabrication paper Pulp bleaching Paper fabrication Pulp and paper integrated Chemical Paint industries Glass Soap Acid, base, salt Rubber Synthetic rubber Petroleum refinery Detergent Ammonia Carbon dioxide Petroleum Lactose Sulphur Pharmaceutical products (vitamins) Manufacturing Precision mechanics, products optical, electronic Fine ceramic Machine industry Metallurgy Foundry Lamination Forging Electroplating Iron and steel plating industry Mining Iron Coal

Water consumption per unit Unit (m3 /unit) (*) 1 tonne product 4–50 1 tonne product 5–25 1 tonne sugar 0.5 – 10.0 1 cow or 2,5 pig 0.5–3.0 1000 L milk 1–10 1000 L milk 2–10 1 tonne margarine 20 1000 L beer 5–20 1 tonne bread 2–4 1000 L soft drinks 2–5 1 tonne product 120–750 1 tonne product 500–600 1 tonne product 25–60 1 tonne product 100–150 1 tonne product 60–130 1 tonne wool 20–70 1 tonne product 20–60 1 tonne hide 20–40 1000 pairs of shoes 5 1 tonne product 15–200 1 tonne product 80–200 1 tonne product 30–250 1 tonne product 200–250 1 employee 110 L/d 1 tonne glass 3–30 1 tonne soap 25–200 1 tonne chlorine 50 1 tonne product 100–150 1 tonne product 500 1 barrel (117 L) 0.2–0.4 1 tonne product 13 1 tonne product 100–130 1 tonne product 60–90 1 tonne product 7–30 1 tonne product 600–800 1 tonne product 8–10 1 tonne product 10–30 1 employee 1 employee 1 employee 1 tonne pig iron 1 tonne product 1 tonne product 1 m3 of solution 1 employee 1 m3 mineral taken 1 tonne coal

20–40 L/d 40 L/d 40 L/d 3–8 8–50 80 1–25 60 L/d 16 2–10

∗ Consumption in m3 per unit produced or L/d per employee Source: CETESB (1976), Downing (1978), Arceivala (1981), Hosang and Bischof (1984), Imhoff & Imhoff (1985), Metcalf & Eddy (1991), Der´ısio (1992)

30

Wastewater characteristics, treatment and disposal SOLIDS IN SEWAGE WATER

SEWAGE

SOLIDS

POLLUTION

WASTEWATER TREATMENT

Figure 2.7. Solids in sewage

undertaking the various laboratory tests, but also to the fact that the results themselves cannot be directly utilised as elements in design and operation. Therefore, many times it is preferable to utilise indirect parameters that represent the character or the polluting potential of the wastewater in question. These parameters define the quality of the sewage, and can be divided into three categories: physical, chemical and biological parameters.

2.2.2 Main characteristics of wastewater Tables 2.13, 2.14 and 2.15 present the main physical, chemical and biological characteristics of domestic sewage.

Table 2.13. Main physical characteristics of domestic sewage Parameter Temperature

Colour Odour

Turbidity

Description

• Slightly higher than in drinking water • Variations according to the seasons of the years (more stable than the air temperature) Influences microbial activity Influences solubility of gases Influences viscosity of the liquid Fresh sewage: slight grey Septic sewage: dark grey or black Fresh sewage: oily odour, relatively unpleasant Septic sewage: foul odour (unpleasant), due to hydrogen sulphide gas and other decomposition by-products • Industrial wastewater: characteristic odours • Caused by a great variety of suspended solids • Fresher or more concentrated sewage: generally greater turbidity

• • • • • • •

Source: Adapted from Qasim (1985)

Wastewater characteristics

31

Table 2.14. Main chemical characteristics of domestic sewage Parameter TOTAL SOLIDS • Suspended • Fixed

• Volatile • Dissolved • Fixed • Volatile • Settleable ORGANIC MATTER

Description Organic and inorganic; suspended and dissolved; settleable • Part of organic and inorganic solids that are non-filterable • Mineral compounds, not oxidisable by heat, inert, which are part of the suspended solids • Organic compounds, oxidisable by heat, which are part of the suspended solids • Part of organic and inorganic solids that are filterable. Normally considered having a dimension less than 10−3 µm. • Mineral compounds of the dissolved solids. • Organic compounds of the dissolved solids • Part of organic and inorganic solids that settle in 1 hour in an Imhoff cone. Approximate indication of the settling in a sedimentation tank. Heterogeneous mixture of various organic compounds. Main components: proteins, carbohydrates and lipids.

Indirect determination

• BOD5

• COD

• Ultimate BOD

• Biochemical Oxygen Demand. Measured at 5 days and

20 ◦ C. Associated with the biodegradable fraction of carbonaceous organic compounds. Measure of the oxygen consumed after 5 days by the microorganisms in the biochemical stabilisation of the organic matter. • Chemical Oxygen Demand. Represents the quantity of oxygen required to chemically stabilise the carbonaceous organic matter. Uses strong oxidising agents under acidic conditions. • Ultimate Biochemical Oxygen Demand. Represents the total oxygen consumed at the end of several days, by the microorganisms in the biochemical stabilisation of the organic matter.

Direct determination

• TOC TOTAL NITROGEN

• Organic nitrogen • Ammonia • Nitrite • Nitrate TOTAL PHOSPHORUS

• Organic phosphorus • Inorganic phosphorus

• Total Organic Carbon. Direct measure of the carbonaceous organic matter. Determined through the conversion of organic carbon into carbon dioxide. Total nitrogen includes organic nitrogen, ammonia, nitrite and nitrate. It is an essential nutrient for microorganisms’ growth in biological wastewater treatment. Organic nitrogen and ammonia together are called Total Kjeldahl Nitrogen (TKN). • Nitrogen in the form of proteins, aminoacids and urea. • Produced in the first stage of the decomposition of organic nitrogen. • Intermediate stage in the oxidation of ammonia. Practically absent in raw sewage. • Final product in the oxidation of ammonia. Practically absent in raw sewage. Total phosphorus exists in organic and inorganic forms. It is an essential nutrient in biological wastewater treatment. • Combined with organic matter. • Orthophosphates and polyphosphates. (Continued )

32

Wastewater characteristics, treatment and disposal

Table 2.14 (Continued ) Parameter pH

ALKALINITY

CHLORIDES OILS AND GREASE

Description Indicator of the acidic or alkaline conditions of the wastewater. A solution is neutral at pH 7. Biological oxidation processes normally tend to reduce the pH. Indicator of the buffer capacity of the medium (resistance to variations in pH). Caused by the presence of bicarbonate, carbonate and hydroxyl ions. Originating from drinking water and human and industrial wastes. Fraction of organic matter which is soluble in hexane. In domestic sewage, the sources are oils and fats used in food.

Source: adapted from Arceivala (1981), Qasim (1985), Metcalf & Eddy (1991)

Table 2.15. Main organisms present in domestic sewage Organism Bacteria

Archaea

Algae

• • • • • • • • • •

Fungi

•

Protozoa

• • • • • •

Viruses

• • •

Helminths

• •

Description Unicellular organisms Present in various forms and sizes Main organisms responsible for the stabilisation of organic matter Some bacteria are pathogenic, causing mainly intestinal diseases Similar to bacteria in size and basic cell components Different from bacteria in their cell wall, cell material and RNA composition Important in anaerobic processes Autotrophic photosynthetic organisms, containing chlorophyll Important in the production of oxygen in water bodies and in some sewage treatment processes In lakes and reservoirs they can proliferate in excess, deteriorating the water quality Predominantly aerobic, multicellular, non-photosynthetic, heterotrophic organisms Also of importance in the decomposition of organic matter Can grow under low pH conditions Usually unicellular organisms without cell wall Majority is aerobic or facultative Feed themselves on bacteria, algae and other microorganisms Essential in biological treatment to maintain an equilibrium between the various groups Some are pathogenic Parasitic organisms, formed by the association of genetic material (DNA or RNA) and a protein structure Pathogenic and frequently difficult to remove in water or wastewater treatment Higher-order animals Helminth eggs present in sewage can cause illnesses

Note: algae are normally not present in untreated wastewater, but are present in the treated effluent from some processes (e.g. stabilisation ponds) Source: Silva & Mara (1979), Tchobanoglous & Schroeder (1985), Metcalf & Eddy (1991), 2003

Wastewater characteristics

33

2.2.3 Main parameters defining the quality of wastewater 2.2.3.1 Preliminaries The main parameters predominantly found in domestic sewage that deserve special consideration are:

• • • • •

solids indicators of organic matter nitrogen phosphorus indicators of faecal contamination

2.2.3.2 Solids All the contaminants of water, with the exception of dissolved gases, contribute to the solids load. In wastewater treatment, the solids can be classified according to (a) their size and state, (b) their chemical characteristics and (c) their settleability: Solids in sewage

• Classification by size and state • Suspended solids (non-filterable) • Dissolved solids (filterable) • Classification by chemical characteristics • Volatile solids (organic) • Fixed solids (inorganic) • Classification by settleability • Settleable suspended solids • Non-settleable suspended solids a)

Classification by size

The division of solids by size is above all a practical division. For convention it can be said that particles of smaller dimensions capable of passing through a filter paper of a specific size correspond to the dissolved solids, while those with larger dimensions and retained by the filter are considered suspended solids. To be more precise, the terms filterable (=dissolved) solids and non-filterable (=suspended) solids are more adequate. In an intermediate range there are the colloidal solids, which are of importance in water treatment, but are difficult to identify by the simple method of paper filtration. Water analysis results based on typical filter papers show that the major part of colloidal solids is separated as filterable (dissolved) solids. Sometimes the term particulate is used to indicate that the solids are present as suspended solids. In this context, expressions as particulate BOD, COD, phosphorus, etc. are used, to indicate that they are linked to suspended solids. In contrast, soluble BOD, COD and phosphorus are associated with dissolved solids.

34

Wastewater characteristics, treatment and disposal DISTRIBUTION OF SOLIDS BY SIZE visible to naked eye

BACTERIAL FLOCS VIRUSES ALGAE, PROTOZOA BACTERIA

COLLOIDAL

DISSOLVED

−6

10

−5

10

−4

10

−3

10

−2

10

10

−1

SUSPENDED

10

0

1

10

2

10

3

10

PARTICLE SIZE (µm)

Figure 2.8. Classification and distribution of solids as a function of size

Figure 2.8 shows the distribution of particles by size. In a general manner, are considered dissolved solids those with a diameter of less than 10−3 µm, colloidal solids those with a diameter between 10−3 and 100 µm and as suspended solids those with a diameter greater than 100 µm. b)

Classification by chemical characteristics

If the solids are submitted to a high temperature (550 ◦ C), the organic fraction is oxidised (volatilised), leaving after combustion only the inert fraction (unoxidised). The volatile solids represent an estimate of the organic matter in the solids, while the non-volatile solids (fixed) represent the inorganic or mineral matter. In summary:
Volatile solids (organic matter) Total solids  Fixed solids (inorganic matter) c)

Classification by settleability

Settleable solids are considered those that are able to settle in a period of 1 hour. The volume of solids accumulated in the bottom of a recipient called an Imhoff Cone is measured and expressed as mL/L. The fraction that does not settle represents the non-settleable solids (usually not expressed in the results of the analysis). Figure 2.9 shows the typical distribution between the various types of solids present in a raw sewage of average composition.

Wastewater characteristics

35

L L L L L

L

L

Figure 2.9. Approximate distribution of the solids in raw sewage (in terms of concentration)

2.2.3.3 Carbonaceous organic matter The organic matter present in sewage is a characteristic of substantial importance, being the cause of one of the main water pollution problems: consumption of dissolved oxygen by the microorganisms in their metabolic processes of using and stabilising the organic matter. The organic substances present in sewage consist mainly of (Pessoa & Jord˜ao, 1982):

• • • •

Protein compounds (≈ 40%) Carbohydrates (≈ 25 to ≈ 50%) Oils and grease (≈ 10%) Urea, surfactants, phenols, pesticides and others (lower quantity)

The carbonaceous organic matter (based on organic carbon) present in the influent sewage to a WWTP can be divided into the following main fractions: Organic matter in sewage

• classification: in terms of form and size • Suspended (particulate) • Dissolved (soluble) • classification: in terms of biodegradability • Inert • Biodegradable In practical terms it is not usually necessary to classify organic matter in terms of proteins, fats, carbohydrates, etc. Besides, there is a great difficulty in determining

36

Wastewater characteristics, treatment and disposal

in the laboratory the various components of organic matter in wastewater, in view of the multiple forms and compounds in which it can be present. As a result, direct or indirect methods can be adopted for the quantification of organic matter:

•

• a)

Indirect methods: measurement of oxygen consumption • Biochemical Oxygen Demand (BOD) • Ultimate Biochemical Oxygen Demand (BODu ) • Chemical Oxygen Demand (COD) Direct methods: measurement of organic carbon • Total Organic Carbon (TOC)

Biochemical Oxygen Demand (BOD)

The main ecological effect of organic pollution in a water body is the decrease in the level of dissolved oxygen. Similarly, in sewage treatment using aerobic processes, the adequate supply of oxygen is essential, so that the metabolic processes of the microorganisms can lead to the stabilisation of the organic matter. The basic idea is then to infer the “strength” of the pollution potential of a wastewater by the measurement of the oxygen consumption that it would cause, that is, an indirect quantification of the potential to generate an impact, and not the direct measurement of the impact in itself. This quantification could be obtained through stoichiometric calculations based on the reactions of oxidation of the organic matter. If the substrate was, for example, glucose (C6 H12 O6 ), the quantity of oxygen required to oxidise the given quantity of glucose could be calculated through the basic equation of respiration. This is the principle of the so-called Theoretical Oxygen Demand (TOD). In practice, however, a large obstacle is present: the sewage has a great heterogeneity in its composition, and to try to establish all its constituents in order to calculate the oxygen demand based on the chemical oxidation reactions of each of them is totally impractical. Besides, to extrapolate the data to other conditions would not be possible. The solution found was to measure in the laboratory the consumption of oxygen exerted by a standard volume of sewage or other liquid, in a predetermined time. It was thus introduced the important concept of Biochemical Oxygen Demand (BOD). The BOD represents the quantity of oxygen required to stabilise, through biochemical processes, the carbonaceous organic matter. It is an indirect indication, therefore, of the biodegradable organic carbon. Complete stabilisation takes, in practical terms, various days (around 20 days or more for domestic sewage). This corresponds to the Ultimate Biochemical Oxygen Demand (BODu ). However, to shorten the time for the laboratory test, and to allow a comparison of the various results, some standardisations were established:

•

the determination is undertaken on the 5th day. For typical domestic sewage, the oxygen consumption on the fifth day can be correlated with the final total consumption (BODu );

Wastewater characteristics

•

37

the test is carried out at a temperature of 20◦ C, since different temperatures interfere with the bacteria’s metabolism, modifying the relation between BOD at 5 days and BOD Ultimate.

The standard BOD is expressed as BOD20 5 . In this text, whenever the nomenclature BOD is used, implicitly the standard BOD is being assumed. The BOD test can be understood in this simplified way: on the day of the sample collection, the concentration of dissolved oxygen (DO) in the sample is determined. Five days later, with the sample maintained in a closed bottle and incubated at 20◦ C, the new DO concentration is determined. This new DO concentration is lower due to the consumption of oxygen during the period. The difference in the DO level on the day zero and day 5 represents the oxygen consumed for the oxidation of the organic matter, being therefore, the BOD5 . Thus, for example, a sample from a water body presented the following results (see Figure 2.10): DO on day 0: 7 mg/L DO on day 5: 3 mg/L BOD5 = 7 − 3 = 4 mg/L

L

L L

Figure 2.10. Example of the BOD20 5 concept

For sewage, some practical aspects require some adaptations. Sewage, having a large concentration of organic matter, consumes quickly (well before the five days) all the dissolved oxygen in the liquid medium. Thus, it is necessary to make dilutions in order to decrease the concentration of the organic matter, such that the oxygen consumption at 5 days is numerically less than the oxygen available in the sample (the sample is lost if, at day 5, the DO concentration is zero, because it will not be possible to know when the zero concentration was reached). Also it is usually necessary to introduce a seed, containing microorganisms, to allow a faster start of the decomposition process. To measure only the carbonaceous oxygen demand, an inhibitor for nitrification (nitrogenous oxygen demand, associated with the oxidation of ammonia to nitrate) can be added. Domestic sewage has a BOD in the region of 300 mg/L, or that is to say, 1 litre of sewage is associated with the consumption of approximately 300 mg of oxygen, in five days, in the process of the stabilisation of the carbonaceous organic matter. The main advantages of the BOD test are related to the fact that the test allows:

• •

an approximate indication of the biodegradable fraction of the wastewater; an indication of the degradation rate of the wastewater;

38

Wastewater characteristics, treatment and disposal

• •

an indication of the oxygen consumption rate as a function of time; an approximate determination of the quantity of oxygen required for the biochemical stabilisation of the organic matter present.

However, the following limitations may be mentioned (Marais & Ekama, 1976):

• • •

• • •

low levels of BOD5 can be found in the case that the microorganisms responsible for the decomposition are not adapted to the waste; heavy metals and other toxic substances can kill or inhibit the microorganisms; the inhibition of the organisms responsible for the oxidation of ammonia is necessary, to avoid the interference of the oxygen consumption for nitrification (nitrogenous demand) with the carbonaceous demand; the ratio of BODu /BOD5 varies with the wastewater; the ratio of BODu /BOD5 varies, for the same wastewater, along the WWTP treatment line; the test takes five days, being not useful for operational control of a WWTP.

Despite of the limitations above, the BOD test continues to be extensively used, partly for historical reasons and partly because of the following points:

• •

the design criteria for many wastewater treatment processes are frequently expressed in terms of BOD; the legislation for effluent discharge in many countries, and the evaluation of the compliance with the discharge standards, is normally based on BOD.

Substantial research has been directed towards the substitution of BOD by other parameters. In the area of instrumentation, there are respirometric equipments that make automated measurements of the oxygen consumption, allowing a reduction in the period required for the test. However, universality has not yet been reached regarding the parameter or the methodology. It is observed that the COD test is being more and more used for design, mathematical modelling and performance evaluation. However, the sanitary engineer must be familiar with the interpretation of the BOD and COD tests and know how to work with the complementary information that they both supply. The present text utilises BOD in items in which the more consolidated international literature is based on BOD, and it uses COD in the items, usually more recent, in which the literature is based more on COD. In this way, it is easier to compare the design parameters presented in this book with international literature parameters. b)

Ultimate Biochemical Oxygen Demand (BODu )

The BOD5 corresponds to the oxygen consumption exerted during the first 5 days. However, at the end of the fifth day the stabilisation of the organic material is still not complete, continuing, though at slower rates, for another period of weeks or days.

Wastewater characteristics

39

Table 2.16. Typical ranges for the BODu /BOD5 ratio Origin High concentration sewage Low concentration sewage Primary effluent Secondary effluent

BODu /BOD5 1.1–1.5 1.2–1.6 1.2–1.6 1.5–3.0

Source: Calculated using the coefficients presented by Fair et al (1973) and Arceivala (1981)

L 5

Figure 2.11. Progression in time of BOD in a sample, showing BOD5 and BOD ultimate

After this, the oxygen consumption can be considered negligible. In this way the Ultimate Biochemical Oxygen Demand corresponds to the oxygen consumption until this time, after what there is no significant consumption, meaning that the organic matter has been practically all stabilised. Figure 2.11 shows the progression of BOD in time, in a sample analysed along various days. For domestic sewage, it is considered, in practical terms, that after 20 days of the test the stabilisation is practically complete. Therefore the BODu can be determined at 20 days. The concept of the test is similar to the standard BOD of 5 days, varying only with the final period of determination of the dissolved oxygen concentration. Table 2.16 presents typical ranges of the conversion factor for BOD5 to BODu (domestic waste). Such a conversion is important, because various sewage treatment processes are designed using a BODu base. Chapter 3 shows how to proceed with this conversion using a specific formula. Various authors adopt the ratio BODu /BOD5 equal to 1.46. This means that, in the case of having a BOD5 of 300 mg/L, the BODu is assumed to be equal to 1.46 × 300 = 438 mg/L.

40 c)

Wastewater characteristics, treatment and disposal Chemical Oxygen Demand (COD)

The COD test measures the consumption of oxygen occurring as a result of the chemical oxidation of the organic matter. The value obtained is, therefore, an indirect indication of the level of organic matter present. The main difference with the BOD test is clearly found in the nomenclature of both tests. The BOD relates itself with the biochemical oxidation of the organic matter, undertaken entirely by microorganisms. The COD corresponds to the chemical oxidation of the organic matter, obtained through a strong oxidant (potassium dichromate) in an acid medium. The main advantages of the COD test are:

• • • • •

the test takes only two to three hours; because of the quick response, the test can be used for operational control; the test results give an indication of the oxygen required for the stabilisation of the organic matter; the test allows establishment of stoichiometric relationships with oxygen; the test is not affected by nitrification, giving an indication of the oxidation of the carbonaceous organic matter only (and not of the nitrogenous oxygen demand).

The main limitations of the COD test are:

•

• •

in the COD test, both the biodegradable and the inert fractions of organic matter are oxidised. Therefore, the test may overestimate the oxygen to be consumed in the biological treatment of the wastewater; the test does not supply information about the consumption rate of the organic matter along the time; certain reduced inorganic constituents could be oxidised and interfere with the result.

For raw domestic sewage, the ratio COD/BOD5 varies between 1.7 and 2.4. For industrial wastewater, however, this ratio can vary widely. Depending on the value of the ratio, conclusions can be drawn about the biodegradability of the wastewater and the treatment process to be employed (Braile & Cavalcanti, 1979):

•

•

•

Low COD/BOD5 ratio (less than 2.5 or 3.0): • the biodegradable fraction is high • good indication for biological treatment Intermediate COD/BOD5 ratio (between 2.5 and 4.0): • the inert (non-biodegradable) fraction is not high • treatability studies to verify feasibility of biological treatment High COD/BOD5 ratio (greater than 3.5 or 4.0): • the inert (non-biodegradable) fraction is high • possible indication for physical–chemical treatment

Wastewater characteristics

41

BODu/BOD5 and COD/BOD5 ratios 6 5

BODu/BOD5

COD/BOD5

Ratio

4 3 2 1 0 Raw

Treated

Raw

Treated

Figure 2.12. Ranges of values of the ratios BODu /BOD5 and COD/BOD5 for raw sewage and biologically treated sewage

The COD/BOD5 ratio also varies as the wastewater passes along the various units of the treatment works. The tendency is for the ratio to increase, owing to the stepwise reduction of the biodegradable fraction, at the same time that the inert fraction remains approximately unchanged. In this way, the final effluent of the biological treatment has values of the COD/BOD5 ratio usually higher than 3.0. d)

Total Organic Carbon (TOC)

In this test the organic carbon is directly measured, in an instrumental test, and not indirectly through the determination of the oxygen consumed, like the three tests above. The TOC test measures all the carbon released in the form of CO2 . To guarantee that the carbon being measured is really organic carbon, the inorganic forms of carbon (like CO2 , HCO− 3 etc) must be removed before the analysis or be corrected when calculated (Eckenfelder, 1980). The TOC test has been mostly used so far in research or in detailed evaluations of the characteristics of the liquid, due to the high costs of the equipment. e) Relationship between the representative parameters of oxygen consumption In samples of raw and treated domestic sewage, the usual ratios between the main representative parameters of oxygen consumption (BODu /BOD5 and COD/BOD5 ) are shown in Figure 2.12. The following comments can be made:

• • •

The ratios can never be lower than 1.0. The ratios increase, from the condition of untreated to biologically treated wastewater. The higher the treatment efficiency, the higher the value of the ratio.

42

Wastewater characteristics, treatment and disposal Table 2.18. Predominant forms of nitrogen in the water Form Molecular nitrogen Organic nitrogen Free ammonia Ammonium ion Nitrite ion Nitrate ion

Formula N2 Variable NH3 NH4 + NO2 − NO3 −

Oxidation state 0 Variable −3 −3 +3 +5

2.2.3.4 Nitrogen In its cycle in the biosphere, nitrogen alternates between various forms and oxidation states, resulting from various biochemical processes. In the aquatic medium, nitrogen can be found in the forms presented in Table 2.18. Nitrogen is a component of great importance in terms of generation and control of the water pollution, principally for the following aspects:

•

•

Water pollution • nitrogen is an essential nutrient for algae leading, under certain conditions, to the phenomenon of eutrophication of lakes and reservoirs; • nitrogen can lead to dissolved oxygen consumption in the receiving water body due to the processes of the conversion of ammonia to nitrite and this nitrite to nitrate; • nitrogen in the form of free ammonia is directly toxic to fish; • nitrogen in the form of nitrate is associated with illnesses such as methaemoglobinaemia Sewage treatment • nitrogen is an essential nutrient for the microorganisms responsible for sewage treatment; • nitrogen, in the processes of the conversion of ammonia to nitrite and nitrite to nitrate (nitrification), which can occur in a WWTP, leads to oxygen and alkalinity consumption; • nitrogen in the process of the conversion of nitrate to nitrogen gas (denitrification), which can take place in a WWTP, leads to (a) the economy of oxygen and alkalinity (when occurring in a controlled form) or (b) the deterioration in the settleability of the sludge (when not controlled).

The determination of the prevailing form of nitrogen in a water body can provide indications about the stage of pollution caused by an upstream discharge of sewage. If the pollution is recent, nitrogen is basically in the form of organic nitrogen or ammonia and, if not recent, in the form of nitrate (nitrite concentrations are normally low). In summary, the distinct forms can be seen in a generalised form presented in Table 2.19 (omitting other sources of nitrogen apart from sewage).

Wastewater characteristics

43

Table 2.19. Relative distribution of the forms of nitrogen under different conditions Condition Raw wastewater

• • • Recent pollution in a water course • • Intermediate stage in the pollution of a water course • • • • Remote pollution in a water course Effluent from a treatment process without nitrification • • Effluent from a treatment process with nitrification • Effluent from a treatment process with nitrification/ denitrification

Prevailing form of nitrogen Organic nitrogen Ammonia Organic nitrogen Ammonia Organic nitrogen Ammonia Nitrite (in lower concentrations) Nitrate Nitrate Ammonia Nitrate Low concentrations of all forms of nitrogen

Note: organic nitrogen + ammonia = TKN (Total Kjeldahl Nitrogen)

In raw domestic sewage, the predominant forms are organic nitrogen and ammonia. Organic nitrogen corresponds to amina groups. Ammonia is mainly derived from urea, which is rapidly hydrolysed and rarely found in raw sewage. These two, together, are determined in the laboratory by the Kjeldahl method, leading to the Total Kjeldahl Nitrogen (TKN). Most of the TKN in domestic sewage has physiological origin. The other forms of nitrogen are usually of lesser importance in the influent to a WWTP. In summary:

• •

TKN = ammonia + organic nitrogen (prevailing form in domestic sewage) TN = TKN + NO2 − + NO3 − (total nitrogen)

The distribution of ammonia in the raw sewage can be represented schematically as shown in Figure 2.13. It is seen that the fraction of the oxidised nitrogen NOx (nitrite + nitrate) is negligible in raw sewage. TKN can be further subdivided in a soluble fraction (dominated by ammonia) and a particulate fraction (associated with the organic suspended solids − nitrogen participates in the constitution of practically all forms of particulate organic matter in sewage). Ammonia exists in solution in the form of the ion (NH4 + ) and in a free form, not ionised (NH3 ), according to the following dynamic equilibrium: NH3 + H+ ↔ NH4 + free ammonia ionised ammonia

(2.6)

44

Wastewater characteristics, treatment and disposal

Figure 2.13. Distribution of nitrogen forms in untreated domestic sewage (adapted from IAWQ, 1995)

The relative distribution has the following values, as a function of the pH values. Distribution between the forms of ammonia

• • •

pH < 8 Practically all the ammonia is in the form of NH4 + pH = 9.5 Approximately 50% NH3 and 50% NH4 + pH > 11 Practically all the ammonia in the form of NH3

In this way it can be seen that, in the usual range of pH, near neutrality, the ammonia present is practically in the ionised form. This has important environmental consequences, because free ammonia is toxic to fish even in low concentrations. The temperature of the liquid also influences this distribution. At a temperature of 25 ◦ C, the proportion of free ammonia relative to the total ammonia is approximately the double compared with a temperature of 15 ◦ C. The following equation allows the calculation of the proportion of free ammonia within total ammonia as a function of temperature and pH (Emerson et al, 1975):

Free NH3 H −1 (%) = 1 + 100.09018+[2729.92/(T+273.20)]−P × 100 Total ammonia

(2.6)

where: T = liquid temperature (◦ C) Application of Equation 2.6 leads to the values of the ammonia distribution presented in Table 2.20 and illustrated in Figure 2.14.

Wastewater characteristics

45

Table 2.20. Proportion of free and ionised ammonia within total ammonia, as a function of pH and temperature T = 15 ◦ C pH 6.50 7.00 7.50 8.00 8.50 9.00 9.50

% NH3 0.09 0.27 0.86 2.67 7.97 21.50 46.41

T = 20 ◦ C

% NH4 + 99.91 99.73 99.14 97.33 92.03 78.50 53.59

T = 25 ◦ C

% NH4 + 99.87 99.60 98.76 96.18 88.84 71.57 44.32

% NH3 0.13 0.40 1.24 3.82 11.16 28.43 55.68

% NH3 0.18 0.57 1.77 5.38 15.25 36.27 64.28

% NH4 + 99.82 99.43 98.23 94.62 84.75 63.73 35.72

NH3/total ammonia [%]

% of free ammonia (NH3) 100 90 T=15°C

80 70

T=20°C

60 50 40

T=25°C

30 20 10 0 6.50

7.50

8.50

9.50

10.50

11.50

pH

Figure 2.14. Percentage of free ammonia (NH3 ) within total ammonia, as a function of pH and temperature

In a watercourse or in a WWTP, the ammonia can undergo subsequent transformations. In the process of nitrification the ammonia is oxidised to nitrite and the nitrite to nitrate. In the process of denitrification the nitrates are reduced to nitrogen gas.

2.2.3.5 Phosphorus Total phosphorus in domestic sewage is present in the form of phosphates, according to the following distribution (IAWQ, 1995):

• •

inorganic (polyphosphates and orthophosphates) – main source from detergents and other household chemical products organic (bound to organic compounds) – physiological origin

Phosphorus in detergents is present, in raw sewage, in the form of soluble polyphosphates or, after hydrolysis, as orthophosphates. Orthophosphates are directly available for biological metabolism without requiring conversion to simpler

46

Wastewater characteristics, treatment and disposal

Figure 2.15. Distribution of phosphorus forms in untreated domestic sewage (IAWQ, 1995)

forms. The forms in which orthophosphates are present in the water are pH dependent, and include PO4 3− , HPO4 2− , H2 PO4 − , H3 PO4 . In typical domestic sewage the prevailing form is HPO4 −2 . Polyphosphates are more complex molecules, with two or more phosphorus atoms. Polyphosphates are converted into orthophosphates by hydrolysis, which is a slow process, even though it takes place in the sewerage collection system itself. Mathematical models for wastewater treatment processes usually consider that both forms of phosphate are represented by orthophosphates since after hydrolysis they will effectively be present as such. Phosphorus in detergents can account for up to 50% of the total phosphorus present in domestic sewage. Another way of fractionating phosphorus in wastewater is with respect to its form as solids (IAWQ, 1995):

•

•

soluble phosphorus (predominantly inorganic) – mainly polyphosphates and orthophosphates (inorganic phosphorus), together with a small fraction corresponding to the phosphorus bound to the soluble organic matter in the wastewater particulate phosphorus (all organic) – bound to particulate organic matter in the wastewater

Figure 2.15 illustrates the fractionation of phosphorus in untreated domestic sewage. The importance of phosphorus is associated with the following aspects:

•

Phosphorus is an essential nutrient for the growth of the microorganisms responsible for the stabilisation of organic matter. Usually domestic sewage

Wastewater characteristics

•

47

has sufficient levels of phosphorus, but a lack may occur in some industrial wastewaters; Phosphorus is an essential nutrient for the growth of algae, eventually leading, under certain conditions, to the eutrofication of lakes and reservoirs.

2.2.3.6 Pathogenic organisms and indicators of faecal contamination a) Pathogenic organisms The list of organisms of importance in water and wastewater quality was presented in Table 2.15. Most of these organisms play various essential roles, mainly related to the transformation of the constituents in the biogeochemical cycles. Biological wastewater treatment relies on these organisms, and this aspect is covered in many parts of this book. Another important aspect in terms of the biological quality of a water or wastewater is that related to the disease transmission by pathogenic organisms. The major groups of pathogenic organisms are: (a) bacteria, (b) viruses, (c) protozoans and (d) helminths. Water-related disease is defined as any significant or widespread adverse effects on human health, such as death, disability, illness or disorders, caused directly or indirectly by the condition, or changes in the quantity or quality of any water (Grabow, 2002). A useful way of classifying the water-related diseases is to group them according to the mechanism by which they are transmitted (water borne, water hygiene, water based, water related). Table 2.21 presents the main four categories, with a summary description and the main preventive strategies to be employed. Table 2.22 details the faecal–oral transmission diseases (water borne and water hygiene), with the main pathogenic agents and symptoms. Faecal–oral diseases are of special interest for the objectives and theme of this book, since they are associated with proper excreta and wastewater treatment and disposal. The number of pathogens present in the sewage of a certain community varies substantially and depends on: (a) socio-economic status of the population; (b) health requirements; (c) geographic region; (d) presence of agroindustries; (e) type of treatment to which the sewage was submitted. b)

Indicator organisms

The detection of pathogenic organisms, mainly bacteria, protozoans and viruses, in a sample of water is difficult, because of their low concentrations. This would demand the examination of large volumes of the sample to detect the pathogenic organisms. The reasons are due to the following factors:

• •

in a population, only a certain fraction suffers from water-borne diseases; in the faeces of these inhabitants, the presence of pathogens may not occur in high proportions;

hygiene create conditions for their transmission • Non-faecal route (cannot be water borne)

• Lack of water and poor

• Infections of the skin or

eyes, due to low availability of water and poor hygiene

(e.g. food, bad hygiene)

• Various faecal-oral routes

• Infections of the

Water hygiene

intestinal tract, due to low availability of water and poor hygiene

Transmission mode • Faecal–oral (pathogen in water is ingested by man or animal)

Description • Ingestion of contaminated water

Mechanism Water borne

Table 2.21. Mechanisms of transmission of water-related infections

• Others (e.g. louse-borne typhus)

trachoma)

• Infectious eye diseases (e.g.

sepsis, scabies, fungal infections)

• Infectious skin diseases (e.g. skin

• • • • • •

•

•

Main diseases Diarrhoeas and dysenteries (amoebic dysentery, balantidiasis, Campylobacter enteritis, cholera, E. coli diarrhoea, giardiasis, cryptosporidiosis, rotavirus diarrhoea, salmonellosis, bacillary dysentery, yersiniosis) Enteric fevers (typhoid, paratyphoid) Poliomyelitis Hepatitis A Leptospirosis Ascariasis Trichuriasis Similar to above (water borne)

and food hygiene

• Promote personal, domestic

quantity

• Supply water in sufficient

Preventive strategy • Protect and treat drinking water • Avoid use of contaminated water

water or bite near water

• Insects which breed in

spends part of its life cycle in a water snail or other aquatic animal

• Pathogen (helminth)

animal

• Insect bites man or

skin or is ingested

• Pathogen penetrates the Schistosomiasis Guinea worm Clonorchiasis Diphyllobothriasis Fasciolopsiasis Paragonimiasis Others Malaria Sleeping sickness Filariasis River blindness Mosquito-borne virus (e.g. yellow fever, dengue) • Others

• • • • • • • • • • • •

Source: Cairncross and Feachem (1990), Heller & M¨oller (1995), van Buuren et al (1995), Heller (1997)

Water related

Water based

(e.g. sprays, nets)

• Adopt individual protection

sites

• Avoid contact with breeding

insects

• Combat intermediate host • Combat insect vectors • Destroy breeding sites of

excreta or wastewater disposal

• Adopt adequate solutions for

excreta

• Protect water sources from

contaminated water

• Avoid contact with

50

Wastewater characteristics, treatment and disposal

Table 2.22. Main water-borne and water hygiene (faecal oral transmission) diseases, according to pathogenic organism Organism

Disease Bacillary dysentery (shigellosis) Campylobacter enteritis

Causal agent Shigella dysenteriae

Symptoms / manifestation Severe diarrhoea

Campylobacter jejuni, Campylobacter coli

Cholera

Vibrio cholerae

Gastroenteritis

Escherichia coli – enteropathogenic Leptospira – various species Salmonella – various species

Diarrhoea, abdominal pain, malaise, fever, nausea, vomiting Extremely heavy diarrhoea, dehydration, high death rate Diarrhoea

Leptospirosis Bacteria Paratyphoid fever

Salmonella

Protozoan

Typhoid fever

Salmonella – various species Salmonella typhi

Amoebic dysentery

Entamoeba histolytica

Giardiasis

Giardia lamblia

Cryptosporidiosis Balantidiasis Infectious hepatitis Respiratory disease

Cryptosporidium Balantidium coli Hepatitis A virus Adenovirus – various types Enterovirus, Norwalk, rotavirus, etc. – various species Enterovirus

Gastroenteritis Viruses Meningitis

Poliomyelitis virus

Poliomyelitis (infantile paralysis) Ascariasis

Ascaris lumbricoides

Trichuriasis

Trichuris trichiura

Helminths

Jaundice, fever Fever, diarrhoea, malaise, headache, spleen enlargement, involvement of lymphoid tissues and intestines Fever, nausea, diarrhoea High fever, diarrhoea, ulceration of small intestine Prolonged diarrhoea with bleeding, abscesses of the liver and small intestine Mild to severe diarrhoea, nausea, indigestion, flatulence Diarrhoea Diarrhoea, dysentery Jaundice, fever Respiratory illness Mild to strong diarrhoea, vomiting Fever, vomiting, neck stiffness Paralysis, atrophy

Pulmonary manifestations, nutritional deficiency, obstruction of bowel or other organ Diarrhoea, bloody mucoid stools, rectal prolapse

Source: Benenson (1985), Tchobanoglous and Schroeder (1985), Metcalf & Eddy (1991)

Wastewater characteristics

• • •

51

after discharge to the receiving body or sewerage system, there is still a high dilution of the contaminated waste; sensitivity and specificity of the tests for some pathogens; broad spectrum of pathogens.

In this sense, the final concentration of pathogens per unit volume in a water body may be considerably low, making detection through laboratory examination highly difficult. This obstacle is overcome through the search for indicator organisms of faecal contamination. These organisms are predominantly non-pathogenic, but they give a satisfactory indication of whether the water is contaminated by human or animal faeces, and, therefore, of its potential to transmit diseases. The organisms most commonly used with this objective are bacteria of the coliform group. The following are the main reasons for the use of the coliform group as indicators of faecal contamination:

•

•

•

•

Coliforms are present in large quantities in human faeces (each individual excretes on average 1010 to 1011 cells per day) (Branco and Rocha, 1979). About 1/3 to 1/5 of the weight of human faeces consist of bacteria from the coliform group. All individuals eliminate coliforms, and not only those who are ill, as is the case with pathogenic organisms. Thus the probability that the coliforms will be detected after the sewage discharge is much higher than with pathogenic organisms. Coliforms present a slightly higher resistance in the water compared with the majority of enteric pathogenic bacteria. This characteristic is important, because they would not be good indicators of faecal contamination if they died faster than pathogenic organisms, and a sample without coliforms could still contain pathogens. On the other hand, if their mortality rate were much lower than that of pathogenic microorganisms, the coliforms would not be useful indicators, since their presence could unjustifiably make suspect a sample of purified water. These considerations apply mainly to pathogenic bacteria, since other microorganisms can present a higher resistance compared to coliforms. The removal mechanisms for coliforms from water bodies, water treatment plants and WWTP are the same mechanisms used for pathogenic bacteria. In this way the removal of pathogenic bacteria is usually associated with the removal of coliforms. Other pathogenic organisms (such as protozoan cysts and helminth eggs), however, can be removed by different mechanisms. The bacteriological techniques for coliform detection are quick and economic compared with those for pathogens.

The indicators of faecal contamination most commonly used are:

• • •

total coliforms (TC) faecal coliforms (FC) or thermotolerant coliforms Escherichia coli (EC)

52

Wastewater characteristics, treatment and disposal

The group of total coliforms (TC) constitutes a large group of bacteria that have been isolated in water samples and in polluted and non polluted soils and plants, as well as from faeces from humans and other warm-blooded animals. This group was largely used in the past as an indicator, and continues to be used in some areas, although the difficulties associated with the occurrence of non-faecal bacteria are a problem (Thoman and Mueller, 1987). There is no quantifiable relation between TC and pathogenic microorganisms. The total coliforms could be understood in a simplified way as “environmental” coliforms, given their possible occurrence in non-contaminated water and soils, thus representing other free-living organisms, and not only the intestinal ones. For this reason, total coliforms should not be used as indicators of faecal contamination in surface waters. However, in the specific case of potable water supply, it is expected that treated water should not contain total coliforms. These, if found, could suggest inadequate treatment, post contamination or excess of nutrients in the treated water. Under these conditions, total coliforms could be used as indicators of the water treatment efficiency and of the integrity of the water distribution system (WHO, 1993). Faecal coliforms (FC) are a group of bacteria predominantly originated from the intestinal tract of humans and other animals. This group encompasses the genus Escherichia and, to a lesser degree, species of Klebsiella, Enterobacter and Citrobacter (WHO, 1993). The test for FC is completed at a high temperature, aiming at suppressing bacteria of non-faecal origin (Thoman and Mueller, 1987). However, even under these conditions, the presence of non-faecal (free-living) bacteria is possible, although in lower numbers compared with the total coliforms test. As a result, even the test for faecal coliforms does not guarantee that the contamination is really faecal. For this reason, recently the faecal coliforms have been preferably denominated thermotolerant coliforms, because of the fact that they are resistant to the high temperatures of the test, but are not necessarily faecal. Whenever in the present book reference is made to faecal coliforms (traditional in the literature and in the environmental legislation in various countries), it should be understood, implicitly, the more appropriate terminology of thermotolerant coliforms. Escherichia coli (EC) is the main bacterium of the faecal (thermotolerant) coliform group, being present in large numbers in the faeces from humans and animals. It is found in wastewater, treated effluents and natural waters and soils that are subject to recent contamination, whether from humans, agriculture, wild animals and birds (WHO, 1993). Its laboratory detection is very simple, principally by recent fluorogenic methods. Different from total and faecal coliforms, E. coli is the only that gives guarantee of exclusively faecal contamination. For this reason, there is a current tendency in using predominantly E. coli as indicator of faecal contamination. However, its detection does not guarantee that the contamination is from human origin, since E. coli can also be found in other animal faeces. There are some types of E. coli that are pathogenic, but this does not invalidate its concept as bacterial indicators of faecal contamination. The detection of faecal contamination, exclusively human, requires the use of complementary biochemical tests, which are not usually undertaken in routine analysis.

Wastewater characteristics

53

Table 2.23. Application of total coliforms, thermotolerant coliforms and E. coli as indicators of faecal contamination Faecal Total (thermotolerant) Item Sample coliform coliforms E. coli Guarantee that the Water bodies reasonably clean Low Reasonable Total contamination Water bodies polluted Reasonable High Total is of faecal by sewage origin Guarantee that Water bodies reasonably clean None None None the faecal Water bodies polluted mainly Reasonable High High contamination by domestic sewage is exclusively human Proportion of Water bodies reasonably clean Variable Variable – E. coli in the Water bodies polluted by Reasonable High – total count domestic sewage to high of coliforms Domestic sewage Very high Very high –

Figure 2.16. Schematic representation of bacteria and indicators of faecal contamination

Figure 2.16 illustrates the relative distribution of the indicator, pathogenic and other forms of bacteria. Table 2.23 synthesises the application of the three groups of indicators discussed above. In sewage, E. coli is the predominant organism within the group of faecal (thermotolerant) coliforms, and the faecal (thermotolerant) coliforms are the predominant group within the total coliforms. For water bodies, when doing the interpretation of the tests for indicators of faecal contamination, it is very important to carry out a sanitary survey of the catchment area. This survey helps in establishing the origin of the faecal contamination (presence of domestic sewage discharges or wastes from animals), complementing the information supplied by the laboratory tests.

54

Wastewater characteristics, treatment and disposal

For the objectives of this book (wastewater treatment), the characterisation of the faecal origin is not so important, since it is already accepted that the wastewater will contain faecal matter and organisms. The indicator organisms are used, in this case, as indicators of the pathogen removal efficiency in the wastewater treatment process. The pathogenic organisms that can be represented are bacteria and viruses, since they are removed by the same mechanisms of the coliform bacteria. Protozoan cysts and helminth eggs, which are mainly removed by physical mechanisms, such as sedimentation and filtration, are not well represented by coliform bacteria as indicators of treatment efficiency. There are various other indicator organisms proposed in the literature, each with its own advantages, disadvantages and applicability. Below some of these organisms are briefly discussed. Faecal streptococci. The group of faecal streptococci comprises two main genera: Enterococcus and Streptococcus. The genus Enterococcus encompasses many species, the majority of them of faecal human origin; however, some species are from animal origin. All Enterococcus present high tolerance to adverse environmental conditions. The genus Streptococcus comprises the species S. bovis and S. equinus, which are abundant in animal faeces. Faecal streptococci seldom multiply in polluted waters, and are more resistant than E. coli and coliform bacteria (WHO, 1993). Because of these characteristics, they have been used as indicators for bathing waters. In the past, the ratio between the values of faecal coliforms and faecal streptococci (FC/FS ratio) was used to give an indication of the origin of the contamination, whether predominantly human or animal. High values of FC/FS would suggest predominantly human contamination, whereas low values of FC/FS would suggest predominantly animal contamination. More recent evidences indicate, however, that these relations are not applicable in a large number of situations, giving unreliable indications about the real origin or the contamination in various catchment areas. Sulphite-reducing clostridia. Clostridium perfringens is the most representative species in this group, being normally present in faeces, although in much smaller numbers than E. coli. However, it is not exclusively of faecal origin and can be derived from other environmental sources. Clostridial spores can survive in water much longer than organisms of the coliform group and will also resist disinfection. Their presence in disinfected waters may indicate deficiencies in treatment and that disinfectant-resistant pathogens could have survived treatment. Because of its longevity, it is best regarded as indicating intermittent or remote contamination. However, false alarms may also result from its detection, which makes it of special value, but not particularly recommended for routine monitoring of water distribution systems (WHO, 1993). Bacteriophages. For the indication of the presence of viruses, bacteriophages may be representative, owing to their similarities with the enteric human viruses. Bacteriophages are specific viruses that infect bacteria, for example the coliphages, which infect E. coli. Coliphages are not present in high numbers in fresh human or animal faeces, but may be abundant in sewage, owing to their fast reproduction rate

Wastewater characteristics

55

resulting from the attack to bacterial cells (Mendon¸ca, 2000). Their significance is as indicators of sewage contamination and, because of their greater persistence compared with bacterial indicators, as additional indicators of treatment efficiency or for groundwater protection. Helminth eggs. For helminths, there are no substituting indicators, and helminth eggs are determined directly in laboratory tests. However, the eggs of nematodes, such as Ascaris, Trichuris, Necator americanus and Ancilostoma duodenale may be used as indicators of other helminths (cestodes, trematodes and other nematodes), which are removed in water and wastewater treatment by the same mechanism (e.g. sedimentation), being thus indicators of treatment efficiency. Helminth eggs are an important parameter when assessing the use of water or treated wastewater for irrigation, in which workers may have direct contact with contaminated water and consumers may eat the irrigated vegetable uncooked or unpeeled. Helminth eggs may be removed by physical operations, such as sedimentation, which takes place, for instance, in stabilisation ponds. Eggs may be viable or non-viable, and viability may be altered by specific disinfection processes. This topic is under constant development, and the present text does not aim to go deeper into specific items, covering only the more general and simplified concepts.

2.2.4 Relationship between load and concentration Before presenting the typical concentrations of the main pollutants in sewage, it is important to be clear about the concepts of per capita, load and constituent concentration. Per capita load represents the average contribution of each individual (expressed in terms of pollutant mass) per unit time. A commonly used unit is grams per inhabitant per day (g/inhab.d). For example, when the BOD contribution is 54 g/inhab.d, it is equivalent to saying that every individual discharges 54 grams of BOD on average, per day. The influent load to a WWTP corresponds to the quantity of pollutant (mass) per unit time. In this way, import relations are load = population × per capita load load (kg/d) =

population (inhab) × per capita load (g/inhab.d) 1000 (g/kg)

(2.7) (2.8)

or load = concentration × flow load (kg/d) = Note: g/m3 = mg/L

concentration (g/m3 ) × flow (m3 /d) 1000 (g/kg)

(2.9) (2.10)

56

Wastewater characteristics, treatment and disposal

The concentration of a wastewater can be obtained through the rearrangement of the same dimensional relations: concentration = load/flow

concentration (g/m3 ) =

load (kg/d) × 1000 (g/kg) flow (m3 /d)

(2.11)

(2.12)

Example 2.2 Calculate the total nitrogen load in the influent to a WWTP, given that:

• •

concentration = 45 mgN/L flow = 50 L/s

Solution: Expressing flow in m3 /d, : Q=

50 L/s × 86400 s/d 1000 L/m3

The nitrogen load is: load =

45 g/m3 × 4320 m3 /d = 194 kgN/d 1000 g/kg

b) In the same works, calculate the total phosphorus concentration in the influent, given that the influent load is 40 kgP/d. concentration =

40 kg/d × 1000 g/kg = 9.3 gP/m3 = 9.3 mgP/L 4320 m3 /d

2.2.5 Characteristics of domestic sewage The typical quantitative physical–chemical characteristics of predominantly domestic sewage in developing countries can be found in a summarised form in Table 2.24. Campos and von Sperling (1996) verified, for essentially domestic sewage in nine sub-catchment areas in the city of Belo Horizonte, Brazil, relationships between per capita BOD load and BOD concentration with the average family income.

Wastewater characteristics

57

Table 2.24. Physical–chemical characteristics of raw domestic sewage in developing countries Per capita load (g/inhab.d) Parameter TOTAL SOLIDS Suspended • Fixed • Volatile Dissolved • Fixed • Volatile Settleable ORGANIC MATTER BOD5 COD BOD ultimate TOTAL NITROGEN Organic nitrogen Ammonia Nitrite Nitrate PHOSPHORUS Organic phosphorus Inorganic phosphorus pH ALKALINITY HEAVY METALS TOXIC ORGANICS

Concentration (mg/L, except pH)

Range 120–220 35–70 7–14 25–60 85–150 50–90 35–60 –

Typical 180 60 10 50 120 70 50 –

Range 700–1350 200–450 40–100 165–350 500–900 300–550 200–350 10–20

Typical 1100 350 80 320 700 400 300 15

40–60 80–120 60–90 6.0–10.0 2.5–4.0 3.5–6.0 ≈0 0.0–0.3 0.7–2.5 0.7–1.0 0.5–1.5 20–40 ≈0 ≈0

50 100 75 8.0 3.5 4.5 ≈0 ≈0 1.0 0.3 0.7 30 ≈0 ≈0

250–400 450–800 350–600 35–60 15–25 20–35 ≈0 0–2 4–15 1–6 3–9 6.7–8.0 100–250 ≈0 ≈0

300 600 450 45 20 25 ≈0 ≈0 7 2 5 7.0 200 ≈0 ≈0

Sources: Arceivala (1981), Jord˜ao & Pessoa (1995), Qasim (1985), Metcalf & Eddy (1991), Cavalcanti et al (2001) and the author’s experience.

The higher the income, the higher is the per capita BOD load and the lower is the BOD concentration (Figure 2.17). Family income is expressed as numbers of minimum salaries. The figures are presented in order to show the large influence of economic status, and not to allow direct calculations, since the economical data are region specific. The typical biological characteristics of domestic sewage, in terms of pathogenic organisms, can be found in Table 2.25.

2.2.6 Characteristics of industrial wastewater 2.2.6.1 General concepts The generalisation of typical industrial wastewater characteristics is difficult because of their wide variability from time to time and from industry to industry.

58

Wastewater characteristics, treatment and disposal Table 2.25. Microorganisms present in raw domestic sewage in developing countries Microorganisms Total coliforms Faecal (thermotolerant) coliforms E. coli Faecal streptococci Protozoan cysts Helminth eggs Viruses

Per capita load (org/inhab.d) 1010 –1013 109 –1012 109 –1012 107 –1010 1.61

Source: Fair et al (1973), Arceivala (1981)

3.2.5.2 The reaeration coefficient K2 In a sample of deoxygenated water, the value of the coefficient K2 can be determined through statistical methods. These methods are based on regression analysis, using either the original Equation 3.10, or some logarithmic transformation of it. The input data are the DO values at various times t. The output data are the saturation concentration Cs and the coefficient K2 . In a water body, however, the experimental determination of K2 is very complex, being outside the scope of the present text. The value of the coefficient K2 has a larger influence on the results of the DO balance than the coefficient K1 , because of the fact that the ranges of variation of K1 are narrower. There are three methods for estimating the value of the coefficient K2 in the river under study:

• • • a)

average tabulated values values as a function of the hydraulic characteristics of the water body values correlated with the flow of the water body

Average tabulated values of K2

Some researchers, studying water bodies with different characteristics, proposed average values for K2 based on a qualitative description of the water body (Table 3.4). Shallower and faster water bodies tend to have a larger reaeration coefficient, due, respectively, to the greater ease in mixing along the depth and the creation of more turbulence on the surface (see Figure 3.11). The values in Table 3.4 can be used in the absence of specific data from the water body. It must be taken into consideration that the values from this table are usually lower than those obtained by the other methods discussed below. However, there are indications that, in some situations, the tabulated values result in better fitting to measured DO data than those obtained from hydraulic formula. b)

K2 values as a function of the hydraulic characteristics of the water body

Other researchers correlated the reaeration coefficient K2 with the hydraulic variables of the water body. Various field techniques were employed in their studies, such as tracers, equilibrium disturbance, mass balance and others.

100

Wastewater characteristics, treatment and disposal

Figure 3.11. Influence of the physical characteristics of the water body on the coefficient K2

The literature presents various formulas, conceptual and empirical, relating K2 with the depth and the velocity of the water body. Table 3.5 and Figure 3.12 present three of the main formulas, with application ranges that are complementary. If there are natural cascades with free water falls, other formulations for the estimation of the atmospheric reaeration may be used. Von Sperling (1987) obtained the following empirical formula, based on the study of some waterfalls in Brazil: Ce = C0 + K.(Cs − C0 ) K = 1 − 1.343.H−0.128 .(Cs − C0 )−0.093

(3.11) (3.12)

where: Ce = effluent (downstream) DO concentration (mg/L) C0 = influent (upstream) DO concentration (mg/L) K = efficiency coefficient (−) Cs = DO saturation concentration (mg/L) H = height of each free fall (m) c)

K2 values correlated with the river flow

Another approach for estimating K2 is through the correlation with the river flow. This can be justified by the fact that the depth and the velocity are intimately associated with flow.

Impact of wastewater discharges to water bodies

101

Table 3.5. Values of the coefficient K2 , according with models based on hydraulic data (base e, 20 ◦ C) Researcher O’Connor & Dobbins (1958)

Formula 3.73.v0.5 H−1,5

Churchill et al (1962)

5.0.v0.97 H−1,67

Owens et al (cited by Branco, 1976)

5.3.v0.67 H−1,85

Application range 0.6 m ≤ H < 4.0 m 0.05 m/s ≤ v < 0.8 m/s 0.6 m ≤ H < 4.0 m 0.8 m/s ≤ v < 1.5 m/s 0.1 m ≤ H < 0.6 m 0.05 m/s ≤ v < 1.5 m/s

Notes:

• v: velocity of the water body (m/s) • H: height of the water column (m) • Ranges of applicability adapted and slightly modified from Covar (EPA, 1985), for simplicity

&

Figure 3.12. Approximate applicability ranges of three hydraulic formulas for estimating K2 (modified from Covar, cited in EPA, 1985)

The procedure is based on the determination of K2 using the hydraulic formula (section b above), for each pair of values of v and H from historical records in the river. Subsequently, a regression analysis is performed between the resulting values of K2 and the corresponding flow values Q. The relation between K2 and Q may be expressed as K2 = m.Qn , where m and n are coefficients. The advantage of this form of expression is that the reaeration coefficient may be calculated for any flow conditions (by interpolation or even some extrapolation), especially minimum flows, independently from depth and velocity values.

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Wastewater characteristics, treatment and disposal

3.2.5.3 Influence of temperature The influence of temperature is felt in two different ways:

• •

an increase in temperature reduces the solubility of oxygen in the liquid medium (decrease of the saturation concentration Cs ) an increase in temperature accelerates the oxygen absorption processes (increase of K2 )

These factors act in opposite directions. The increase in K2 implies an increase in the reaeration rate. However, a reduction in the saturation concentration corresponds to a decrease in the oxygen deficit D, resulting in a reduction in the reaeration rate. The overall influence on the reaeration rate depends on the magnitude of each variation but is frequently not substantial. The influence of the temperature on the saturation concentration is discussed in Section 3.2.7k. The influence of temperature on the reaeration coefficient can be expressed in the traditional form (Equation 3.13):

K2T = K220 .θ(T−20)

(3.13)

where: K2T = K2 at a temperature T(d−1 ) K220 = K2 at a temperature T = 20 ◦ C (d−1 ) T = liquid temperature (◦ C) θ = temperature coefficient (−) A value frequently used for the temperature coefficient θ is 1.024.

3.2.6 The DO sag curve 3.2.6.1 Mathematical formulation of the model In 1925 the researchers Streeter and Phelps established the mathematical bases for the calculation of the dissolved oxygen profile in a water course. The structure of the model proposed by them (known as the Streeter–Phelps model) is classical within environmental engineering, setting the basis for all the other more sophisticated models that succeeded it. For the relatively simple situation in which only the deoxygenation and the atmospheric reaeration are taken into account in the DO balance, the rate of change of the oxygen deficit with time can be expressed by the following differential equation, originated from the interaction of the deoxygenation and reaeration equations previously seen: Rate of change of the DO deficit = DO consumption − DO production

(3.14)

Impact of wastewater discharges to water bodies

103

dD = K1 .L − K2 .D dt

(3.15)

Integration of this equation leads to: Dt =

K1 .L0 .(e−K1 .t − e−K2 .t ) + D0 .e−K2 .t K2 − K 1

(3.16)

This is the general equation that expresses the variation of the oxygen deficit as a function of time. The DO concentration curve (DOt or Ct ) can be obtained directly from this equation, knowing this: DOt = Cs − Dt

(3.17)

Thus: 

K1 .L0 Ct = Cs − .(e−K1 .t − e−K2 .t ) + (Cs − C0 ).e−K2 .t K2 − K 1

(3.18)

In the DO sag curve, one point is of fundamental importance: the point in which the DO concentration reaches its lowest value. This is called critical time, and the DO concentration, the critical concentration. The knowledge of the critical concentration is very important, because it is based on it that the need and efficiency of the wastewater treatment will be established. The treatment must be implemented with a BOD removal efficiency which is sufficient to guarantee that the critical DO concentration is higher than the minimum value required by legislation (standard for the water body). The DO sag curve as a function of time (or of the distance) is S-shaped, as shown in Figure 3.13. In the curve, the main points are identified: the DO concentration in the river and the critical DO concentration.

3.2.6.2 Model equations a) DO concentration and deficit in the river immediately after mixing with the wastewater C0 =

Qr .DOr + Qw .DOw Qr + Q w

(3.19)

D0 = Cs − C0

(3.20)

where: C0 = initial oxygen concentration, immediately after mixing (mg/L) D0 = initial oxygen deficit, immediately after mixing (mg/L)

104

Wastewater characteristics, treatment and disposal DISSOLVED OXYGEN PROFILE WASTEWATER WATER COURSE

Cs Cr

Do

DO (mg/L) Co

Dc Cc

to

tc

time (d) or distance (km)

Figure 3.13. Characteristic points in the DO sag curve

Cs = oxygen saturation concentration (mg/L) Qr = river flow upstream of the wastewater discharge (m3 /s) Qw = wastewater flow (m3 /s) DOr = dissolved oxygen concentration in the river, upstream of discharge (mg/L) DOw = dissolved oxygen concentration in the wastewater (mg/L) It can be observed that the value of C0 is obtained through the weighted average between the flows and the DO levels in the river and the wastewater. b) BOD5 and ultimate BOD concentrations in the river immediately after mixing with the wastewater

BOD50 =

(Q r .BODr + Q w .BODw ) Qr + Q w

L 0 = BOD50 .K T =

(Q r .BODr + Q w .BODw ) .K T Qr + Q w

(3.21)

(3.22)

where: BOD50 = BOD5 concentration, immediately after mixing (mg/L) L0 = ultimate oxygen demand (BODu ), immediately after mixing (mg/L) BODr = BOD5 concentration in the river (mg/L) BODw = BOD5 concentration in the wastewater (mg/L) KT = coefficient for transforming BOD5 to the ultimate BODu (−)

Impact of wastewater discharges to water bodies

105

L

L

L

L

Figure 3.14. Relation between the critical time and the terms (L0 /D0 ) and (K2 /K1 )

KT =

BODu 1 = BOD5 1 − e−5.K1

(3.23)

The value of L0 is also obtained through the weighted average between the flows and the biochemical oxygen demands of the river and of the wastewater. c)

DO profile as a function of time 

K1 .L0 Ct = Cs − .(e−K1 .t − e−K2 .t ) + (Cs − C0 ).e−K2 .t K2 − K1

(3.24)

In the event that a negative DO concentration (Ct < 0) is calculated, even though mathematically possible, there is no physical meaning. In this case, anaerobic conditions (DO = 0 mg/L) have been reached and the Streeter−Phelps model is no longer valid. d)

Critical time (time when the minimum DO concentration occurs)    K2 1 D0 .(K2 − K1 ) tc = .ln . 1− K2 − K 1 K1 L0 .K1

(3.25)

The following situations can occur when using the critical time formula, depending on the relation between (L0 /D0 ) and (K2 /K1 ) (see Figure 3.14):

•

L0 /D0 > K2 /K1 Critical time is positive. From the mixing point there will be a decrease in the DO concentration, leading to a critical deficit that is higher than the initial deficit.

106

Wastewater characteristics, treatment and disposal

•

•

•

e)

L0 /D0 = K2 /K1 Critical time is equal to zero, that is, it occurs exactly in the mixing point. The initial deficit is equal to the critical deficit. The water course has a good regenerating capacity for the discharge received, and will not suffer a drop in DO level. L0 /D0 < K2 /K1 Critical time is negative. This indicates that, from the mixing point, the dissolved oxygen concentration tends to increase. The initial deficit is the largest observed deficit. In terms of DO, the water course presents a selfpurification capacity that is higher than the degeneration capacity of the wastewater. In practical terms, the critical time can be considered equal to zero, with the occurrence of the lowest DO values at the mixing point. K2 /K1 = 1 The application of the critical time formula leads to a mathematical indetermination. The limit when K2 /K1 tends to 1 leads to a critical time equal to 1/K1 .

Critical deficit and concentration of dissolved oxygen Dc =

K1 .L 0 .e K 1 .tc K2

Cc = Cs − Dc f)

(3.26)

(3.27)

BOD removal efficiency required in the wastewater treatment

The Streeter–Phelps model still permits the calculation of the maximum allowable BOD load of the sewage, which will lead to the critical DO concentration being equal to the minimum permitted by the legislation. Such procedure involves some iterations because, for each alteration of the maximum permissible load, there is a modification of the critical time. However, in a real situation, with more than one discharge point, this approach becomes not very practical. What is usually done its to consider BOD removal efficiencies which are compatible with the existing or available wastewater treatment processes, and to recalculate the DO profile for each new condition. The most economic situation is usually that in which the minimum DO concentration is only marginally higher than the minimum permitted by legislation.

3.2.7 Input data for the DO model The following input data are necessary for the utilisation of the Streeter–Phelps model (see Figure 3.15):

• •

river flow, upstream of the discharge (Qr ) wastewater flow (Qw )

Impact of wastewater discharges to water bodies

107

Figure 3.15. Input data required for the Streeter–Phelps model

• • • • • • • • • • a)

dissolved oxygen in the river, upstream of the discharge (DOr ) dissolved oxygen in the wastewater (DOw ) BOD5 in the river, upstream of the discharge (BODr ) BOD5 of the wastewater (BODw ) deoxygenation coefficient (K1 ) reaeration coefficient (K2 ) velocity of the river (v) travelling time (t) saturation concentration of DO (Cs ) minimum dissolved oxygen permitted by legislation (DOmin )

River flow (Qr)

The flow of the receiving body is a variable of extreme importance in the model, having a large influence on the simulation results. Therefore it is necessary to obtain the most precise flow value, whenever possible. The use of the DO model can be with any of the following flows, depending on the objectives:

• • •

flow observed in a certain period mean flow (annual average, average in the rainy season, average in the dry season) minimum flow

The observed flow in a certain period is used for model calibration (adjusting the model coefficients), so that the simulated data are as close as possible to the observed (measured) data in the water body during the period under analysis. The mean flow is adopted when the simulation of the average prevailing conditions is desired, such as during the year, rainy months or dry months. The minimum flow is utilised for the planning of catchment areas, the evaluation of the compliance with environmental standards of the water body and for the allocation of pollutant loads. Therefore, the determination of the required efficiencies

108

Wastewater characteristics, treatment and disposal

for the treatment of various discharges must be determined in the critical conditions. These critical conditions in the receiving body occur exactly in the minimum flow period, when the dilution capacity is lower. The critical flow must be calculated from the historical flow measurement data from the water course. The analysis of methods to estimate minimum flows is outside the scope of the present text, being well covered in hydrology books. Usually a minimum flow with a return period of 10 years and a duration of the minimum of 7 days (Q7,10 ), is adopted. This can be understood as a value that may repeat itself in a probability of every 10 years, consisting of the lowest average obtained in 7 consecutive days. Therefore, in each year of the historical data series the 365 average daily flows are analysed. In each year a period of 7 days is selected, which resulted in the lowest average flow (average of 7 values). With the values of the lowest 7-day average for every year, an statistical analysis is undertaken, allowing interpolation or extrapolation of the value for a return period of 10 years. Adoption of the 10-year return period in the Q7,10 concept leads to small flows and frequently to the requirement of high BOD removal efficiencies, the cost of which should always be borne in mind, especially in developing countries. For these countries, probably a shorter return period would be more realistic, especially considering that the current condition is probably already of a polluted river. Another approach is the utilisation of percentiles, such as a 90%ile value (Q90 ). In this concept, 90% of the flow values are greater than the critical flow, and only 10% are lower than it. This approach usually leads to critical flows that are greater than those based on Q7,10 . Under any flow conditions, the utilisation of the concept of the specific discharge (L/s.km2 ) is helpful. Knowing the drainage area at the discharge point and adopting a value for the specific discharge, the product of both leads to the flow of the water course. The specific discharge values can vary greatly from region to region, as a function of climate, topography, soil, etc., For Q7,10 conditions, the following ranges are typical: (a) arid regions: probably less than 1,0 L/s.km2 ; (b) regions with good water resources availability: maybe higher than 3,0 L/s.km2 ; and (c) in intermediate regions: values close to 2.0 L/s.km2 . b)

Wastewater flow (Qw)

Wastewater flows considered in DO modelling are usually average flows, without coefficients for the hour and day of highest consumption. The sewage flow is obtained through conventional procedures, using data from population, per capita water consumption, infiltration, specific contribution (in the case of industrial wastes), etc. The calculation is detailed in Chapter 2. c)

Dissolved oxygen in the river, upstream of the discharge point (DOr)

The dissolved oxygen level in a water body, upstream of a waste discharge, is a result of the upstream activities in the catchment area.

Impact of wastewater discharges to water bodies

109

Ideally, historical data should be used in this analysis. When doing so, coherence is required: if the simulation is for a dry period, only samples pertaining to the dry period should be considered. In case that it is not possible to collect water samples at this point, the DO concentration can be estimated as a function of the approximate pollution level of the water body. If there are few indications of pollution, a DOr value of 80% to 90% of the oxygen saturation value (see item k below) can be adopted. In the event that the water body is already well polluted upstream of the discharge, a sampling campaign is justified, or even an upstream extension of the boundaries of the studies should be considered, in order to include the main polluting points. In such a situation, the value of DOr will be well below the saturation level. d)

Dissolved oxygen in the wastewater (DOw)

In sewage, the dissolved oxygen levels are normally nihil or close to zero. This is due to the large quantity of organic matter present, implying a high consumption of oxygen by the microorganisms. Therefore the DO of raw sewage is usually adopted as zero in the calculations. In case that the wastewater is treated, the following considerations could be made:

• • •

•

•

Primary treatment. Primary effluents can be assumed as having DO equal to zero. Anaerobic treatment. Anaerobic effluents also have DO equal to zero. Activated sludge and biofilm reactors. Effluents from these systems undergo a certain aeration at the effluent weir on the secondary sedimentation tanks, enabling DO to increase to 2 mg/L or slightly more. If the discharge outfall is long, this oxygen could be consumed as a result of the remaining BOD from the treatment. Facultative or maturation ponds. Effluents from facultative or maturation ponds can have during day time DO levels close to saturation, or even higher, due to the production of pure oxygen by the algae. At night the DO levels decrease. For the purpose of the calculations, DOw values around 4 to 6 mg/L can be adopted. Effluents subjected to final reaeration. Treatment plant effluents may be subject to a final reaeration stage, in order to increase the level of dissolved oxygen. A simple system is composed by cascade aeration, made up of a sequence of steps in which there is a free fall of the liquid. DO values may raise a few milligrams per litre, depending on the number and height of the steps. Sufficient head must be available for the free falls. Gravity aeration should not be used directly for anaerobic effluents, due to the release of H2 S in the gas-transfer operation. Section 11.10 presents the methodology for calculating the increase in DO.

110

Wastewater characteristics, treatment and disposal Table 3.6. BOD5 as a function of the water body characteristics River condition Very clean Clean Reasonably clean Doubtful Bad

BOD5 of the river (mg/L) 1 2 3 5 >10

Source: Klein (1962)

e)

BOD5 in the river, upstream of discharge (BODr)

BOD5 in the river, upstream of the discharge, is a function of the wastewater discharges (point or diffuse sources) along the river down to the mixing point. The same considerations made for DOr about sampling campaigns and the inclusion of upstream polluting points are also valid here. Klein (1962) proposed the classification presented in Table 3.6, in the absence of specific data. f)

BOD5 in the wastewater (BODw)

The BOD5 concentration in raw domestic sewage has an average value in the order of 300 mg/L. The BOD can also be estimated through the quotient between the BOD load (calculated from the population and the per capita BOD contribution) and the wastewater flow (domestic sewage + infiltration). For more details, see Section 2.2.5. In case there are industrial discharges of importance, particularly from agroindustries and others with high content of organic matter in the effluent, they must be included in the calculation. These values can be obtained by sampling or through literature data. See Section 2.2.6. For a treated wastewater, of course the BOD removal efficiency must be taken into account, since treatment is the main environmental control measure to be adopted. In this case, the BOD5 in the wastewater is: 

BODtw

 E = 1− ·BODrw 100

(3.28)

where: BODtw = BOD5 of the treated wastewater (mg/L) BODrw = BOD5 of the raw wastewater (mg/L) E = BOD5 removal efficiency of the treatment (%) Table 4.9 presents typical ranges of BOD removal efficiency from various wastewater treatment systems. An overview of these systems can be found in

Impact of wastewater discharges to water bodies

111

Chapter 4. Various other chapters of this book are dedicated to the detailed description of these systems. g)

Deoxygenation coefficient (K1 )

The deoxygenation coefficient can be obtained following the criteria presented in Section 3.2.4.2. It must be noted that water bodies that receive biologically treated wastewater have a lower value of K1 (see Table 3.3). For liquid temperatures different from 20 ◦ C, the value of K1 must be corrected (seer Section 3.2.4.3). h)

Reaeration coefficient (K2 )

The reaeration coefficient can be obtained following the procedures outlined in Section 3.2.5.2. For liquid temperatures different from 20 ◦ C, the value of K2 must be corrected (see Section 3.2.5.3). i)

Velocity of the water body (v)

The velocity of the liquid mass in the water course may be estimated using the following methods:

• • • •

direct measurement in the water course data obtained from flow-measuring points use of hydraulic formulas for open channels correlation with flow

In DO simulations that can be done under any flow conditions, obtaining the velocity through the last two methods is more convenient. In other words, it is important that the velocity is coherent with the flow under consideration, since in dry periods the velocities are usually lower, with the opposite occurring in wet periods. The hydraulic formulas are presented in pertinent literature. The most adequate friction factor should be chosen as a function of the river bed characteristics (see Chow, 1959). The flow-correlation method should follow a methodology similar to the one described in Item 3.2.5.2c for the reaeration coefficient. The model to be obtained can have the form v = cQd , where c and d are coefficients obtained from regression analysis. j)

Travel time (t)

In the Streeter–Phelps model, the theoretical travel time that a particle takes to complete a certain river reach is a function of the velocity and the distance. This is because the model assumes a plug-flow regime and does not consider the effects of dispersion. Therefore, knowing the distances and having determined the velocities in each reach, the residence time is obtained directly from the relation: t=

d v · 86, 400

(3.29)

112

Wastewater characteristics, treatment and disposal

where: t = travel (residence) time (d) d = distance (m) v = velocity in the water body (m/s) 86,400 = number of seconds per day (s/d) k) DO saturation concentration (C s ) The saturation concentration of the oxygen can be calculated based on theoretical considerations, or through the use of empirical formulas. The value of Cs is a function of water temperature and altitude:

•

•

The increase in temperature reduces the saturation concentration (the greater agitation of molecules in the water tends to make the dissolved gases pass to the gas phase) The increase in altitude reduces the saturation concentration (the atmospheric pressure is lower, thus exerting a lower pressure for the gas to be dissolved in the water).

There are some empirical formulas in the literature (the majority based on regression analysis) that directly supply the value of Cs (mg/L) as a function of, for example, the temperature T (◦ C). A formula frequently employed is (P¨opel, 1979): Cs = 14.652 − 4.1022 × 10−1 .T + 7.9910 × 10−3 .T2 − 7.7774 × 10−5 .T3 (3.30) The influence of the altitude can be computed by the following relation (Qasim, 1985):   Cs  Altitude fH = = 1− (3.31) Cs 9450 where: fH = correction factor for altitude, for the DO saturation concentration (−) Cs = DO saturation concentration at the altitude H (mg/L) Altitude = altitude (m above sea level) Salinity also affects the solubility of oxygen. The influence of dissolved salts can be computed by the following empirical formula (P¨opel, 1979): γ = 1 − 9 × 10−6 · Csal where: γ = solubility reduction factor (γ = 1 for pure water) Csal = dissolved salts concentration (mg Cl− /L)

(3.32)

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Table 3.7. Saturation concentration for oxygen in clean water (mg/L) Altitude (m) ◦

Temperature ( C) 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

0 11.3 11.1 10.8 10.6 10.4 10.2 10.0 9.7 9.5 9.4 9.2 9.0 8.8 8.7 8.5 8.4 8.2 8.1 7.9 7.8 7.6

500 10.7 10.5 10.2 10.0 9.8 9.7 9.5 9.2 9.0 8.9 8.7 8.5 8.3 8.2 8.1 8.0 7.8 7.7 7.5 7.4 7.2

1000 10.1 9.9 9.7 9.5 9.3 9.1 8.9 8.7 8.5 8.4 8.2 8.0 7.9 7.8 7.6 7.5 7.3 7.2 7.1 7.0 6.8

1500 9.5 9.3 9.1 8.9 8.7 8.6 8.4 8.2 8.0 7.9 7.7 7.6 7.4 7.3 7.2 7.1 6.9 6.8 6.6 6.6 6.4

Table 3.7 presents the saturation concentrations for oxygen in clean water at different temperatures and heights. l) Minimum allowable dissolved oxygen concentration in the water body (DOmin ) The minimum levels of dissolved oxygen that need to be maintained in the water body are stipulated by the legislation applicable in the country or region. In the absence of specific legislation, it is usual to try to maintain DO concentrations in the water body equal to or above 5.0 mg/L.

3.2.8 Measures to control water pollution by organic matter When analysing the possible pollution control strategies for a water body, it is fundamental to have a regionalised view of the catchment area as a whole, aiming at reaching the desired water quality, instead of treating the problems as isolated points. When a regional focus is employed, a great variety of alternative strategies becomes available, normally leading to lower costs and greater safety. An adequate organisational and institutional structure is essential. Among the main control measures, there are:

•

wastewater treatment

114

Wastewater characteristics, treatment and disposal

• • • • a)

flow regularisation in the water body aeration of the water body aeration of the treated wastewater allocation of other uses for the water body

Wastewater treatment

Individual or collective sewage treatment before discharge is usually the main and often the only control strategy. However, its possible combination with some of the other presented strategies should be analysed, aiming at obtaining a technically favourable solution at the lowest cost. Wastewater treatment is the main alternative analysed in the present book. b)

Flow regularisation of the water body

This alternative generally consists of building an upstream dam, in order to augment the low flow under critical conditions. The most attractive option is to include multiple uses for the dam, such as irrigation, hydroelectric power generation, recreation, water supply and others. Another positive aspect is that the effluent from the dams can contain higher levels of dissolved oxygen because of the aeration at the effluent weir. It must be remembered that the implementation of dams is a delicate topic from an environmental point of view. Also, if the upstream catchment area is not properly protected, the reservoir can become a point of localised pollution and risks of eutrophication. c)

Aeration of the water body

Another possibility is to provide aeration in the water body at a point downstream of the discharge, maintaining the DO concentrations above the minimum allowable. An advantage of this alternative resides in the fact that the assimilative capacity of the water course can be totally used in periods of high flow and the aeration can be limited to dry periods only. This is a form of collective treatment and involves the distribution of the costs between the various beneficiaries. The following aeration methods can be employed:

• • • • •

diffused-air aeration surface (mechanical) aeration aeration at weirs turbine aeration injection by pressure

Besides this, natural waterfalls can contribute significantly to the DO increase (see Section 3.2.5.2, equations 3.11 and 3.12). d)

Aeration of the treated wastewater

At the effluent weir of the WWTP, after satisfaction of the oxygen demand, the effluent can suffer a simple aeration, usually by means of weirs. These devices

Impact of wastewater discharges to water bodies

115

can increase the DO concentration in the order of some milligrams per litre (1 to 3 mg/L) contributing to the fact that, already at the mixing point, a slightly higher DO concentration is achieved. In anaerobic effluents, however, aeration must be avoided because it causes the release of hydrogen sulphide, which causes problems of bad odours. e)

Allocation of other uses for the water body

In case it is not possible (mainly for economic reasons) to control the polluting discharges in order to preserve the water quality as a function of the intended uses of the water body, these uses could be re-evaluated in the river or in selected reaches. Therefore, it could be necessary to attribute less noble uses for a certain reach of the river, due to the unfeasibility of implementing the control measure at the desired level. The allocation of uses for the water body should be carried out in such a way as to optimise regional water resources, aiming at their various uses (Arceivala, 1981). Example 3.2 The city and the industry from the general example in Chapter 2 (Section 2.2.7) discharge together their effluents into a water course. The catchment area upstream does not present any other significant discharges. Downstream of the discharge point the stream travels a distance of 50 km until it reaches the main river. In this downstream reach there are no other significant discharges. Main data:

•

Wastewater characteristics (values obtained from the mentioned example, for year 20 of operation): – Average wastewater flow: 0.114 m3 /s – BOD concentration: 341 mg/L • Catchment area characteristics: – Drainage area upstream of the discharge point: 355 km2 – Specific discharge of the water body (minimum flow per unit area of the basin) 2.0 L/s.km2 • Water body characteristics: – Altitude: 1,000 m – Water temperature: 25 ◦ C – Average depth: 1.0 m – Average velocity: 0.35 m/s Assume all other necessary data. Required items:

• • •

Calculate and plot the DO sag curve until the stream joins the main river Present wastewater treatment alternatives for the pollution control of the water body Calculate and plot the DO sag curves for the alternatives analysed

116

Wastewater characteristics, treatment and disposal Example 3.2 (Continued)

Solution: Determination of the input data (raw wastewater) a) River flow (Qr ) Minimum specific discharge: Qrspec = 2.0 L/s.km2 Drainage basin area: A = 355 km2 Qr = Qrspec .A = 2.0 L/s.km2 × 355 km2 = 710 L/s = 0.710 m3 /s b) Wastewater flow (Qw ) Qw = 0.114 m3 /s (stated in the problem) c) Dissolved oxygen in the river (DOr ) Considering that the water body does not present significant discharges, adopt the DO concentration upstream of the discharge as 90% of the saturation value. Saturation concentration: Cs = 7.5 mg/L (25 ◦ C, 1,000 m of altitude) (see item j below) DOr = 0.9 × Cs = 0.9 × 7.5 mg/L = 6.8 mg/L d) Dissolved oxygen in the sewage (DOw ) DOw = 0.0 mg/L (adopted) e) Biochemical oxygen demand in the river (BODr ) From Table 3.6, for a clean river: BODr = 2.0 mg/L f ) Biochemical oxygen demand of the wastewater (BODw ) BODw = 341 mg/L (stated in the problem) g) Deoxygenation coefficient (K1 ) As laboratory tests were not possible, K1 is adopted as an average value from the literature (raw sewage – see Table 3.3): K1 = 0.38 d−1 (20 ◦ C, base e) Correction of K1 for a temperature of 25 ◦ C (Equation 3.8): K1T = K120C ·θ(T−20) = 0.38 × 1.047(25−20) = 0.48d−1

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117

Example 3.2 (Continued) h) Reaeration coefficient (K2 ) Depth of the water body: H = 1.0 m Velocity of the water body: v = 0.35 m/s Formula to be used, according with the applicability range (see Table 3.5 and Figure 3.11): O’Connor and Dobbins formula: K2 = 3.73·

v 0.5 (0.35 m/s)0.5 = 3.73· = 2.21d−1 (20 ◦ C, base e) 1.5 H (1.0 m)1.5

Correction for the temperature of 25 ◦ C (Equation 3.13): K2T = K220C θ(T−20) = 2.21 × 1.024(25−20) = 2.49 d−1 i) Travel time Velocity of the water body: v = 0.35 m/s Travel distance: d = 50,000 m The travel time to arrive at the confluence with the main river is (Equation 3.29): t=

50, 000 m d = = 1.65 d v.86, 400 0.35 m/s.86,400 s/d

j) Saturation concentration of dissolved oxygen (Cs ) Water temperature: T = 25 ◦ C Altitude: 1,000 m From Table 3.6: Cs = 7.5 mg/L l) Minimum allowable dissolved oxygen (DOmin ) DOmin = 5.0 mg/L (adopted)

118

Wastewater characteristics, treatment and disposal Example 3.2 (Continued)

Summary:

Fig Ex 3.2d Input data for the example. Raw wastewater. Calculation of the output data – raw wastewater a) Oxygen concentration at the mixing point (C0 ) From Equation 3.19: C0 =

Qr .DOr + Qw .DOw 0.710 × 6.8 + 0.114 × 0.0 = 5.9 mg/L = Qr + Q w 0.710 + 0.144

The dissolved oxygen deficit is (see Equation 3.20): D0 = Cs − C0 = 7.5 − 5.9 = 1.6 mg/L b) Ultimate BOD concentration at the mixing point (L0 ) The transformation factor BOD5 to BOD ultimate is given by Equation 3.23: KT =

1 BODu 1 = = 1.10 = BOD5 1 − exp(−5.K1 ) 1 − exp(−5 × 0.48)

The BOD5 at the mixing point is obtained from Equation 3.21: BOD50 = =

(Qr .BODr + Qw .BODw ) Qr + Q w (0.710 × 2.0 + 0.114 × 341) = 49 mg/L 0.710 + 0.114

The ultimate BOD at the mixing point is obtained from Equation 3.22: L0 = BOD50 .KT = 49 × 1.10 = 54 mg/L

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119

Example 3.2 (Continued) c) Critical time (tc ) From Equation 3.25:

   1 K2 D0 .(K2 − K1 ) .ln · 1− K2 − K 1 K1 L0 .K1    1 2.49 1.6.(2.49 − 0.48) = .ln · 1− = 0.75d 2.49 − 0.48 0.48 54 × 0.48

tc =

The critical distance is obtained from the critical time and the velocity: dc = t.v.86,400 = 0.75 × 0.35 × 86,400 = 22,680 m = 22.7 km d) Critical concentration of the dissolved oxygen (DOc ) The critical deficit is given by Equation 3.26: K1 0.48 Dc = .L0 .e−K1 .tc = .54.e−0.48 × 0.75 = 7.2mg/L K2 2.49 The critical concentration is given by Equation 3.27: DOc = Cs − Dc = 7.5 − 7.2 = 0.3 mg/L Environmental control measures need to be adopted, since there are DO concentrations lower than the minimum allowable (DOmin = 5.0 mg/L). In case a negative value of DO concentration had been calculated, one should always keep in mind that negative concentrations have no physical meaning. The Streeter–Phelps model would be no longer valid under these conditions (from the moment when DO = 0 mg/L), and the calculation and the graph must be interrupted at this point. However, even in this case the model played an important role, since the requirement for control measures was identified. e) DO sag curve Along the water course, in the absence of specific data, it is assumed that the dilution by natural contributions (direct drainage) is counterbalanced by the BOD load occasionally distributed along the reach (diffuse pollution). In case there were significant tributaries or sewage discharges downstream, the water body should be subdivided into new reaches. It is an essential condition of the Streeter–Phelps model that each reach is homogeneous. From Equation 3.24:   K1 .L0 −K2 .t −K1 .t −K2 .t Ct = Cs − .(e −e ) + (Cs − C0 ).e K2 − K 1   0.48 × 54 = 7.5 − .(e−0.48 × t − e−2.49 × t ) + (7.5 − 5.9).e−2.49 × t 2.49 − 0.48

120

Wastewater characteristics, treatment and disposal Example 3.2 (Continued) For various values of t: d (km)

t (d)

Ct (mg/L)

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

0.00 0.17 0.33 0.50 0.66 0.83 0.99 1.16 1.32 1.49 1.65

5.9 3.1 1.5 0.6 0.3 0.3 0.5 0.8 1.1 1.5 1.9

It can be observed that in practically all the distance, the DO is below the minimum allowable level of 5.0 mg/L. The DO profile can be visualised in Figure 3.16. If a negative DO concentration had occurred, the model would stop being used at the point when DO became less than zero, and the negative values should not be reported or plotted. DO PROFILE: RAW WASTEWATER 8 7 6 5 DO 4 (mg/L) 3 2 1 0

DOmin

0

10

20

30 distance (km)

40

50

60

DO profile in the river. Raw sewage.

Calculation of the output data – treated wastewater After the confirmation of the need for wastewater treatment, the different alternatives of BOD removal efficiencies must be investigated. The concept of treatment level (primary, secondary) used in this example is covered in Chapter 4. For the sake of simplicity, in this example it is assumed that the domestic and industrial wastewaters are mixed and treated together, at the same plant and, therefore, with the same removal efficiency. Other approaches are possible, involving different plants and treatment efficiencies if the domestic and industrial effluents are separated.

Impact of wastewater discharges to water bodies

121

Example 3.2 (Continued) a) Alternative 1: Primary treatment – Efficiency 35% From Equation 3.28, the BOD of the treated wastewater is:     35 E BODtw = BODrw . 1 − = 341. 1 − = 222 mg/L 100 100 The new coefficient K1 (treated wastewater at primary level) can be obtained from Table 3.3, and is adopted in this example as: K1 = 0.35 d−1 (T = 20 ◦ C) K1 = 0.44 d−1 (after correction for T = 25 ◦ C using Equation 3.8) The remaining input data are the same. The calculation sequence is also the same. The calculated and plotted DO values are shown in item d. The critical DO concentration (2.8 mg/L) occurs at a distance of 22.1 km. The minimum allowable value (5.0 mg/L) continues not to be complied with in a large part of the river reach. The efficiency of the proposed treatment is insufficient. Therefore a higher efficiency must be adopted, associated with secondary treatment level. b) Alternative 2: Secondary treatment – Efficiency 65% All secondary-level sewage treatment processes reach a BOD removal efficiency of at least 65%, even the simplest ones. In this item, sewage treatment by UASB (Upflow Anaerobic Sludge Blanket) reactors is analysed. The effluent BOD from the treatment plant is:   65 BODtw = 341. 1 − = 119 mg/L 100 The new coefficient K1 (treated wastewater at secondary level) can be obtained from Table 3.3, and is adopted in this example as: K1 = 0.18 d−1 (T = 20 ◦ C) K1 = 0.23 d−1 (T = 25 ◦ C) It was assumed that the effluent DO from the treatment plant is zero (0.0 mg/L), since the effluent is anaerobic. If a different treatment process is adopted and the effluent has higher levels of DO in the effluent, this must be taken into consideration. Naturally, if only anaerobic reactors are adopted, aeration of the effluent must not be practised since hydrogen sulphide may be released into the atmosphere. The calculated DO values and the graph of the DO profile are presented in item d.

122

Wastewater characteristics, treatment and disposal Example 3.2 (Continued)

Along the whole length of the water course the DO values are above the minimum allowable concentration (the critical DO is 5.4 mg/L, greater than the minimum allowable of 5.0 mg/L). In this way, from the viewpoint of the receiving body, this alternative is satisfactory. Existing BOD discharge standards are not analysed here. These standards vary from country to country or from region to region and they can be taken into consideration when applicable. In the present case, the BOD of the discharge is 119 mg/L. In the case that the environmental agency establishes discharge standards for BOD of, say, 25 mg/L, these standards will not be satisfied in this alternative. Under certain conditions, environmental agencies relax the discharge standard, provided that the standard for the receiving body standard is satisfied. Assuming that the environmental agency has accepted this alternative of 65% BOD removal efficiency, which has been shown to be sufficient in terms of DO, there is no need to investigate other alternatives of greater removal efficiencies, which probably would have higher costs. The most economic situation is usually that in which the critical DO is only marginally greater than the minimum allowable DO. This aspect is of great importance for developing countries. Similarly, there is no need to analyse efficiencies lower than 65%, since this is already on the lower boundary of typical efficiencies for secondary treatment level. If the efficiency of 65% had been unsatisfactory, new efficiencies should be tried in a sequentially increasing way, until the receiving body standard is reached. c) Summary The alternative to be adopted is alternative 2 – sewage treatment at a secondary level, with a BOD removal efficiency of 65%. The DO concentrations in the water body for the three scenarios are presented below. DO concentration (mg/L) d (km)

t (d)

E = 0%

E = 35%

E = 65%

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

0.00 0.17 0.33 0.50 0.66 0.83 0.99 1.16 1.32 1.49 1.65

5.9 3.1 1.5 0.6 0.3 0.3 0.5 0.8 1.1 1.5 1.9

5.9 4.3 3.5 3.0 2.8 2.8 3.0 3.1 3.4 3.6 3.8

5.9 5.6 5.5 5.4 5.4 5.4 5.4 5.5 5.5 5.6 5.7

Impact of wastewater discharges to water bodies

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Example 3.2 (Continued) DO PROFILE - DIFFERENT SCENARIOS 8 7 6 5 DO 4 (mg/L) 3 2 1 0 0

10

20

30

40

50

distance (km) E = 0%

E = 35%

E = 65%

Domin

DO profiles for three different BOD removal efficiencies in the wastewater treatment.

The values above were obtained using a spreadsheet. Small differences in decimals may occur, depending on the criteria employed for rounding the values of the intermediate calculations, especially if they are performed using calculators.

3.3 CONTAMINATION BY PATHOGENIC MICROORGANISMS 3.3.1 Introduction One of the most important aspects of water pollution is that related with public health, associated with water-borne diseases. This topic, including the main pathogens of interest and the concept of indicator organisms of faecal contamination, is discussed in Chapter 2. A water body receiving the discharge of sewage may incorporate into itself a wide range of pathogenic organisms. This fact may not generate a direct impact on the aquatic organisms themselves, but may affect some of prevailing uses of the water, such as potable water supply, irrigation and bathing. Therefore, it is very important to know the behaviour of the pathogenic organisms in the water body, starting from the discharge point until places where water is likely to be used. It is known that most of these agents have optimal conditions for their growth and reproduction in the human intestinal tract. Once submitted to the adverse conditions that prevail in the water body, they tend to decrease in number, characterising the so-called decay. In Chapter 2 it was seen that the bacteria of the coliform group are used as indicators of faecal contamination; that is, they indicate if the water has been contaminated by faeces and, as a result, if it presents a potential for having pathogens

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Table 3.8. Important factors that contribute to bacterial decay in water bodies Physical factors • solar light (ultraviolet radiation) • temperature (values in water are usually lower than those in human bodies • adsorption • flocculation • sedimentation

• • • •

Physical-chemical factors osmotic effects (salinity) pH chemical toxicity redox potential

Biological and biochemical factors • lack of nutrients • predation • competition

and therefore transmitting diseases. The present item covers the qualitative and quantitative relations associated with the decay of the coliforms in water bodies. It is assumed that this decay represents, with a certain safety, an indication of the behaviour of the pathogens (especially bacteria) discharged into the water body.

3.3.2 Bacterial decay kinetics 3.3.2.1 Intervening factors Coliforms and other microorganisms of intestinal origin present a natural mortality when exposed to environmental conditions that are different from the previously preponderant conditions inside the human system, which were ideal for their development and reproduction. Table 3.8 lists important factors that contribute to the bacterial decay in water bodies (Arceivala, 1981; EPA, 1985; Thomann and Mueller, 1987). These factors may act simultaneously and with different degrees of importance.

3.3.2.2 Kinetics of bacterial decay The bacterial mortality rate is generally estimated by Chick’s law, according to which, the higher the concentration of bacteria, the higher is the decay rate (firstorder reaction): dN = −Kb .N dt where: N = number of coliforms (organisms /100 ml) Kb = coefficient of bacterial decay (d−1 ) t = time (d)

(3.33)

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Table 3.9. Formulas for the calculation of the coliform concentrations in water bodies

Hydraulic regime Plug flow (e.g.: rivers) Completely mixed (e.g.: lakes)

Coliform concentration N (organisms/100ml) N = N0 .e−Kb .t

Scheme

N=

N0

1+Kb .t

N0 = number of coliforms in the influent (organisms/100 ml). In plug-flow reactors, coliforms at time t=0 N = number of coliforms after time t (organisms/100 ml) Kb = coefficient of bacterial decay (d−1 ) t = time (d)

The formula to calculate the coliform concentration after a time t depends on the hydraulic regime of the water body. Rivers are usually represented as plug-flow reactors, while reservoirs are frequently represented as completely-mixed reactors. Depending on the characteristics of the water body, the formulas shown in Table 3 can be used. For completely-mixed reactors, the time t corresponds to the detention time, given by: t = V/Q. The concentration of the coliforms at any point in the reactor is the same, coinciding with the effluent concentration.

3.3.2.3 Bacterial decay coefficient Values of Kb obtained in various studies in fresh water vary within a wide range. Typical values, however, are close to (Arceivala, 1981; EPA, 1985; Thomann and Mueller, 1987): Kb = 0.5 to 1.5 d−1 (base e, 20 ◦ C)

Typical value ≈ 1.0 d−1

The effect of temperature on the decay coefficient can be formulated as: KbT = Kb20 .θ(T−20)

(3.34)

where: θ = temperature coefficient (−) A typical value for θ can be 1.07 (Castagnino, 1977; Thomann and Mueller, 1987), though there is a great variation in the data presented in the literature.

126

Wastewater characteristics, treatment and disposal Table 3.10. Main processes for the removal of pathogenic organisms in wastewater treatment Type

Process Maturation ponds Land infiltration Chlorination Ozonisation Ultraviolet radiation Membranes

Natural

Artificial

Note: for a description of the process – see Chapter 4

3.3.3 Control of the contamination by pathogenic organisms The best measure to control contamination of a water body by pathogenic organisms from sewage is through their removal at the wastewater treatment stage. However, this approach is not practised throughout the world. In various countries there is systematic disinfection of the sewage treatment effluent, while in others disinfection is only carried out in the potable water treatment. However, in any case, approaches that preserve the defined uses of the water body should be adopted. The wastewater treatment processes usually applied are very efficient in the removal of suspended solids and organic matter, but generally insufficient for the removal of pathogenic microorganisms. In spite of the great importance of this item in developing countries, it has not yet received due consideration. Table 4.9 in Chapter 4 lists the coliform removal efficiencies obtained in the main wastewater treatment systems. It should be always remembered that the coliforms are not a direct indication of the presence of pathogens, and they may represent only those organisms that have similar decay (or removal) mechanisms and similar (or higher) mortality rates. Protozoan cysts and helminth eggs are removed by different mechanisms (e.g. sedimentation) and are not well represented by coliforms. Even though removal efficiencies of 90% shown in Table 4.9 may seem high, it should be borne in mind that, when dealing with coliforms, much higher efficiencies are generally necessary in order to have low concentrations in the water body, as a result of the very high concentrations in the raw sewage. High coliform removal efficiencies can be obtained by the processes listed in Table 3.10, which are further detailed in Chapter 4. The processes listed above are capable of reaching coliform removal efficiencies of 99.99% or more. Frequently the coliform removal efficiency is expressed in a logarithmic scale, according to: Removal efficiencies Log units 1 2 3 4

Percentage (%) 90 99 99.9 99.99

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127

For instance, a coliform concentration, which is reduced from 107 organisms/ 100 ml to 104 organisms/100 ml, is reduced in 3 orders of magnitude, or 99.9%. If the logarithms of the concentrations are calculated, the reduction is from 7 to 4 units, in other words, 3 log units. Coliform concentrations are frequently represented in terms of the order of magnitude (powers of 10) or in their logarithms, considering their great variability and the uncertainty in more precise numerical values, and because coliform data usually tend to follow a log-normal distribution. The following formulas relate the efficiency expressed as percentage removal with log units removed. Efficiency (%) = (N0 − N)/N0 = 100 × (1 − 10−log.units removed )

(3.35)

Log units removed = −log10 [1 − (Efficiency (%)/100)]

(3.36)

Not all countries or regions have coliform standards for the water body. When existent, they vary as a function of the water use and a number of local aspects. Values are usually situated around 102 to 103 faecal (thermotolerant) coliforms per 100 ml. Example 3.3 Calculate the concentration profile of faecal (thermotolerant) coliforms in the river of Example 3.2. Calculate the coliform removal efficiency necessary in the wastewater treatment, so that the river presents a coliform concentration lower than 103 CF/100 ml. Data: • river flow: Qr = 0.710 m3 /s • wastewater flow: Qw = 0.114 m3 /s • water temperature: T = 25 ◦ C • travel distance: d = 50 km • velocity of the water: v = 0.35 m/s Solution: a) Faecal coliform concentration in the raw sewage Adopt a faecal coliform concentration of Nrw = 1 × 107 org/100 mL in the raw wastewater (see Chapter 2). b) Faecal coliform concentration in the wastewater–river mixture, after the discharge Assume that the river is clean upstream of the discharge, with a negligible concentration of coliforms (Nr = 0 organisms/100 mL).

128

Wastewater characteristics, treatment and disposal Example 3.3 (Continued)

The concentration in the mixing point is calculated by a weighted average with the flows: N0 =

Qr .Nr + Qw .Nrw 0.710 × 0 + 0.114 × 1 × 107 = Qr + Q w 0.710 + 0.114

= 1.38 × 106 org/100 mL c) Concentration profile along the distance The faecal coliform concentration is calculated by the equation for plug flow (rivers), presented in Table 3.9. Adopting Kb = 1.0 d−1 (20 ◦ C), the value for the temperature of 25 ◦ C is obtained: KbT = Kb20 .(T−20) = 1.0 × 1.07(25−20) = 1.40 d−1 The concentrations as a function of time are calculated from: N = N0 .e−Kb .t = 1.38 × 106 .e−1.4.t Varying t, the values of Nt are obtained. The correspondence between distance and time is given by: d = v.t = (0.35 m/s × 86,400 s/d).t/(1000 m/l2 m The table and graph below present Nt for various values of t and d: d (km)

t (d)

Nt (organisms/100 mL)

0.0 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0

0.00 0.17 0.33 0.50 0.66 0.83 0.99 1.16 1.32 1.49 1.65

1.38 × 106 1.09 × 106 8.69 × 105 6.89 × 105 5.47 × 105 4.34 × 105 3.44 × 105 2.73 × 105 2.17 × 105 1.72 × 105 1.36 × 105

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129

Example 3.3 (Continued) FC x 10−6 (organisms/100 mL)

FAECAL COLIFORM PROFILE - RAW WASTEWATER 1.4 1.2 1 0.8 0.6 0.4 0.2 0

10

20

30

40

50

distance (km)

In spite of the considerable decrease along the travel distance, the concentrations are still very high and far greater than the desired value of 103 organisms/ 100 mL. d) Maximum allowable concentration in the wastewater At the discharge point, the faecal coliform concentration needs to be less than 1,000 organisms/100 mL. Using the equation for the concentration in the mixing point, the maximum desirable concentration in the treated wastewater is obtained. N0 =

Qr .Nr + Qw .Ntw 0.710 × 0 + 0.114 × Ntw = 1, 000 = Qr + Q w 0.710 + 0.114

Ntw = 7,228 organisms/100 mL e) Required removal efficiency of faecal coliforms in the wastewater treatment The required efficiency is: E=

1.0 × 107 − 7, 228 = 0.9993 = 99.93% 1.0 × 107

In log units, the removal efficiency is: Log units removed = −log10 [1 − (E (%)/100)] = − log10 [1 − 0.9993] = 3.15 log units Therefore, the high efficiency of 99.93% (3.15 log units) for the removal of faecal coliforms in the wastewater treatment will be required. Such a high efficiency is not usually reached in the conventional treatment processes, requiring a specific stage for coliform removal (see Table 3.10).

130

Wastewater characteristics, treatment and disposal Example 3.4

Calculate the concentration of the faecal coliforms in a reservoir with a volume of 5,000,000 m3 . The reservoir receives, together, a river and a sewage discharge, both with the same characteristics as in Example 3.3. Calculate the necessary coliform removal efficiency in the wastewater treatment, so that the reservoir has faecal (thermotolerant) coliform concentrations less than or equal to 1000 FC/100 ml. Data: • river flow: Qr = 0.710 m3 /s • wastewater flow: Qw = 0.114 m3 /s • water temperature: T = 25 ◦ C Solution: a) Faecal coliform concentration in the raw sewage Nrw = 1 × 107 organisms/100 mL (same as in Example 3.3). b) Faecal coliform concentration in the wastewater–river mixture N0 = 1.38 × 106 organisms/100 mL (same as in Example 3.3) c) Detention time in the reservoir Total influent flow to the reservoir: Q = Qr + Qw = 0.710 + 0.114 = 0.824 m3 /s t=

V 5,000, 000 m3 = = 70.2 d Q (0.824 m3 /s) × (86.400 s/d)

d) Coliform concentration in the reservoir Assuming a complete-mix model and a Kb value of 1.4 d−1 (equal to Example 3.3, for T = 25 ◦ C), the concentrations of coliforms in the reservoir and in the reservoir effluent are given by (see equation in Table 3.9): N=

N0 1.38 × 106 = = 13, 900 organisms/100 mL 1 + Kb .t 1 + 1.4 × 70.2

= 1.39 × 104 organisms/100 mL This value is above the desired standard of 1,000 organisms/100 mL.

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131

Example 3.4 (Continued) e) Maximum allowable concentration in the wastewater Using the same equation for completely-mixed reactors: N=

N0 N0 = 1, 000 = 1 + Kb .t 1 + 1.4 × 70.2

N0 = 99,280 organisms/100 mL = 9.93 × 104 organisms/100 mL At the sewage–river mixing point, the concentration must be 99.280 organisms/100 mL. Using the equation for the concentration in the mixture (weighted averages), the maximum desirable concentration in the sewage is obtained. N0 =

Qr .Nr + Qw .Nw 0.710 × 0 + 0.114 × Nw = 99.280 = Qr + Q w 0.710 + 0.114

Nw = 717.603 organisms/100 ml = 7.18 × 105 organisms/100 ml f ) Required efficiency for coliform removal in the wastewater treatment E=

1.0 × 107 − 7.18 × 105 = 0.928 = 92.8% 1.0 × 107

This efficiency is lower than the efficiency required in Example 3.3 but this is due to the high detention time in the reservoir (70.2 days) compared with the reduced time in the river (1.65 days). If both systems had the same detention time, the plug-flow reactor (river) would have been more efficient compared with the completely-mixed reactor (reservoir).

3.4 EUTROPHICATION OF LAKES AND RESERVOIRS 3.4.1 The eutrophication process Eutrophication is the excessive growth of aquatic plants, either planktonic, attached or rooted, at such levels as to cause interference with the desired uses of the water body (Thomann and Mueller, 1987). As discussed in this chapter, the main stimulating factor is an excessive level of nutrients in the water body, principally nitrogen and phosphorus. In this chapter, the water bodies under consideration are lakes and reservoirs. The process of eutrophication can also occur in rivers, though this is less frequent, owing to the environmental conditions being less favourable for the growth of algae and other plants, because of factors such as turbidity and high velocities. The following description illustrates the possible sequence of the eutrophication process in a water body, such as a lake or reservoir (see Figure 3.16). The level of eutrophication is usually associated with the predominant land use and occupation in the catchment area.

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Wastewater characteristics, treatment and disposal

Figure 3.16. Sequence of the eutrophication process in a lake or reservoir. Association between land use and eutrophication.

a)

Occupation by woods and forests

A lake situated in a catchment area occupied by woods and forests usually presents a low productivity; that is to say, there is little biological activity of production (synthesis) in it. Even in these natural conditions and in the absence of human

Impact of wastewater discharges to water bodies

133

interference, the lake tends to accumulate solids that settle, which form a layer of sludge at the bottom. With the decomposition of the settled material, there is a certain increase, still incipient, of the level of nutrients in the liquid mass. As a result, there is a progressive increase in the population of aquatic plants and, in consequence, of the other organisms situated at a higher level in the food chain. In the catchment area, the larger part of the nutrients is retained within a nearly closed cycle. The plants die and, in the soil, are decomposed, releasing nutrients. In a region of woods and forests, the infiltration capacity of the rainwater into the soil is high. The nutrients then percolate into the soil, where they are absorbed by the roots of the plants, making part again of their composition and closing the cycle. The input of nutrients to the water body is small. The water body still presents a low trophic level. b)

Agricultural occupation

The removal of natural vegetation from the catchment area for agricultural use generally leads to an intermediate stage in the deterioration process of the water body. The crops planted in the basin are harvested and transported for human consumption, probably outside the catchment area. With this, there is a removal of nutrients that is not naturally compensated, causing a break in the internal cycle. To compensate this removal and to make the agriculture more intensive, fertilisers containing high levels of nitrogen and phosphorus are added artificially. The farmers aim at guaranteeing a high production and thus add high quantities of N and P, frequently greater than the assimilative capacity of the plants. The substitution of woods by plants for agricultural purposes can also cause a reduction in the infiltration capacity of the soil. Therefore the nutrients, already added in excess, are less retained and run off the soil until they eventually reach a lake or reservoir. The increase in the nutrient level in the water body causes a certain increase in the number of algae and, in consequence, of the other organisms located at higher levels in the food chain, culminating with the fish. This relative increase in the productivity of the water body can even be welcome, depending on its intended uses, as would be the case, for instance, in aquaculture. The balance between the positive and negative aspects will depend, to a large extent, on the nutrient assimilative capacity of the water body. c)

Urban occupation

If an agricultural or forest area in a catchment area is substituted by urban occupation, a series of consequences should take place, this time at a faster rate.

•

Silting. The implementation of housing developments implies land movement for the works. Urbanisation also reduces the water infiltration capacity into the soil. The soil particles tend to be transported to the lower parts of the catchment area until they reach the lake or reservoir. In these water bodies, they tend to settle, owing to the low horizontal velocities and turbulence. The sedimentation of the soil particles causes silting and reduces the

134

•

•

Wastewater characteristics, treatment and disposal net volume of the water body. The settled material also serves as a support medium for the growth of rooted plants of larger dimensions (macrophytes) near the shores. In spite of some ecological advantages (e.g. physical retention of pollutants, reduction of sediment resuspension, refuge for fishes and macroinvertebrates), these plants cause an evident deterioration in the visual aspect of the water body. Urban stormwater drainage. Urban drainage transports a far greater load of nutrients in comparison with the other types of occupation of the catchment area. This nutrient input contributes to a rise in the level of algae in the reservoir. Sewage. The greatest deterioration factor, however, is associated with wastewater originating from urban activities. The wastewater contains nitrogen and phosphorus present in faeces and urine, food remains, detergents and other by-products of human activity. The N and P contribution from sewage is much higher than the contribution originating from urban drainage.

Therefore, there is a great increase in the input of N and P onto the lake or reservoir, bringing as a result an elevation in the population of algae and other plants. Depending on the assimilative capacity of the water body, the algal population can reach very high values, bringing about a series of problems, which are described in the subsequent item. In a period with high sunshine (light energy for photosynthesis), the algae can reach superpopulations and be present at massive concentrations at the surface layer. This surface layer hinders the penetration of light energy for the lower layers in the water body, causing the death of algae situated in these regions. The death of these algae brings in itself a series of other problems. These events of superpopulation of algae are called algal blooms.

3.4.2 Problems of eutrophication The following are the main undesired effects of eutrophication (Arceivala, 1981; Thomann and Mueller, 1987; von Sperling, 1994):

•

•

Recreational and aesthetic problems. Reduction of the use of water for recreation, bathing and as a general tourist attraction because of the: • frequent algal blooms • excessive vegetation growth • disturbances with mosquitoes and insects • occasional bad odours • occasional fish mortality Anaerobic conditions in the bottom of the water body. The increase in productivity of the water body causes a rise in the concentration of heterotrophic bacteria, which feed on the organic matter from algae and other dead microorganisms, consuming dissolved oxygen from the liquid medium. In the bottom of the water body there are predominantly anaerobic conditions, owing to the sedimentation of organic matter and the small penetration of oxygen, together with the absence of photosynthesis (absence

Impact of wastewater discharges to water bodies

•

•

•

•

• • •

• •

135

of light). With the anaerobiosis, reducing conditions prevail, leading to compounds and elements being present in a reduced state: • iron and manganese are found in a soluble form, which may bring problems with the water supply; • phosphate is also found in a soluble form, and may represent an internal source of phosphorus for algae; • hydrogen sulphide may also causes problems of toxicity and bad odours. Occasional anaerobic conditions in the water body as a whole. Depending on the degree of bacterial growth, during periods of total mixing of the liquid mass (thermal inversion) or in the absence of photosynthesis (night time), fish mortality and the reintroduction of reduced compounds from the bottom onto the whole liquid mass could occur, leading to a large deterioration in the water quality. Occasional fish mortality. Fish mortality could occur as a result of: • anaerobiosis (mentioned above) • ammonia toxicity. Under conditions of high pH (frequent during periods of high photosynthetic activity), ammonia may be present in significant amounts in its free form (NH3 ), toxic for the fish, instead of the ionised form (NH4+ ), which is non-toxic. Greater difficulty and increase in the costs of water treatment. The excessive presence of algae substantially affects the treatment of water abstracted from a lake or reservoir, due to the necessity of: • removal of the algae themselves • colour removal • taste and odour removal • higher consumption of chemical products • more frequent filter backwashings Problems with industrial water supply. Elevation in the costs of industrial water supply due to reasons similar to those already mentioned, and also to the presence of algae deposits in cooling waters. Water toxicity. Impairment of water for human and animal supply because of the presence of toxic secretions from cyanobacteria (cyanotoxins). Alteration in the quality and quantity of commercial fish. Reduction in navigation and transport capacity. The excessive growth of rooted macrophytes interferes with navigation, aeration and transport capacity of the water body. Negative interference in the equipment for energy generation (macrophytes in turbines). Gradual disappearance of the lake. As a result of eutrophication and silting, there is an increase in the accumulation of material and vegetation, and the lake becomes progressively shallower until it disappears. This tendency of disappearing (conversion to swamps or marshes) is irreversible, although usually extremely slow. With human interference, the process can accelerate abruptly. In case there is no control in the source or the dredging of the sediments, the water body could disappear relatively quickly.

136

Low Low Normally saturated

Low or absent Very low Normally saturated

Normally saturated

Low

Oxygen dynamics of the lower layer

Impairment of multiple uses

Adapted from Vollenweider (cited by Salas and Martino, 1991)

Low

Normally saturated

Oligotrophic Low Low

Ultraoligotrophic Very low Low

Item Biomass Fraction of green algae and/or cyanobacteria Macrophytes Production dynamics Oxygen dynamics at the upper layer

Table 3.11. Trophic characterisation of lakes and reservoirs

Variable

Variable below saturation

Variable Intermediate Variable around supersaturation

Mesotrophic Intermediate Variable

Trophic level

Below saturation to complete absence High

High or low High Frequently supersaturated

Eutrophic High High

Low High, unstable Very unstable, from supersaturation to absence Very unstable, from supersaturation to absence Very high

Hypereutrophic Very high Very high

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137

3.4.3 Trophic levels With the objective of characterising the stage of eutrophication of the water body, allowing the undertaking of preventative and/or corrective measures, it is interesting to adopt a classification system. In a simplified way, there are the following trophic levels:

• • •

oligotrophic (clear lakes with a low productivity) mesotrophic (lakes with an intermediate productivity) eutrophic (lakes with a high productivity, compared with the natural basic level)

This classification can be further detailed, with the inclusion of other trophic levels, such as: ultraoligotrophic, oligotrophic, oligomesotrophic, mesotrophic, mesoeutrophic, eutrophic, eupolitrophic and hypereutrophic (from lowest to highest productivity). A qualitative classification between the main trophic levels may be as presented in Table 3.11. The quantification of the trophic level is, however, more difficult, especially for tropical lakes. Von Sperling (1994) presents a collection of various references, in terms of total phosphorus concentration, chlorophyll a and transparency, which shows the large amplitude in the ranges proposed by various authors. Besides this, the cited reference presents other possible indices to be used, always with the safeguard of the difficulty in generalising the data from one water body to another. It should be kept in mind that tropical water bodies present a larger capacity of phosphorus assimilation in comparison with water bodies in temperate climates. An interpretation of the synthesis reported by von Sperling may be as presented in Table 3.12 in terms of the total phosphorus concentration. The establishment of the trophic levels based only on phosphorus is due to a mathematical modelling convenience. In the same way that in the other water pollution topics covered in the book, representative variables, such as dissolved oxygen (pollution by organic matter) and coliforms (contamination by pathogens),

Table 3.12. Approximate range of values of total phosphorus for the main trophic levels Trophic level Ultraoligotrophic Oligotrophic Mesotrophic Eutrophic Hypereutrophic

Total phosphorus concentration in the reservoir (mgP/m3 ) > 10: limited by phosphorus small lakes, with a predominance of point sources: N/P ∼1 cm)

Main removal mechanisms Retention of the solids with dimensions greater than the spacing between the bars

Suspended solids (> ∼1 µm)

Sedimentation

Separation of the particles with a density greater than the sewage

Dissolved solids (< ∼1 µm) BOD in suspension (particulate BOD) (> ∼1 µm)

Adsorption

Retention on the surface of biomass flocs or biofilms

Sedimentation

Separation of the particles with a density greater than the sewage

Adsorption

Retention on the surface of biomass flocs or biofilms

Hydrolysis

Conversion of the BOD in suspension into soluble BOD by means of enzymes, allowing its stabilisation

Stabilisation

Utilisation by biomass as food, with conversion into gases, water and other inert compounds.

Adsorption

Retention on the surface of biomass flocs or biofilms

Stabilisation

Utilisation by biomass as food, with conversion into gases, water and other inert compounds. Separation of pathogens with larger dimensions and density greater than the sewage

Soluble BOD (< ∼1 µm)

Pathogens

Screening

Larger dimensions and/or with protective layer (protozoan cysts and helminth eggs)

Sedimentation

Filtration

Retention of pathogens in a filter medium with adequate pore size

Lower dimensions (bacteria and viruses)

Adverse environmental conditions

Temperature, pH, lack of food, competition with other species, predation

Ultraviolet radiation

Radiation from the sun or artificial

Disinfection

Addition of a disinfecting agent, such as chlorine (Continued )

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Wastewater characteristics, treatment and disposal

Table 4.3 (Continued ) Pollutant Nitrogen

Subdivision Main removal mechanisms Organic Ammonification Conversion of organic nitrogen into nitrogen ammonia Ammonia

Phosphorus

Nitrification

Conversion of ammonia into nitrite, and the nitrite into nitrate, by means of nitrifying bacteria

Bacterial assimilation

Incorporation of ammonia into the composition of bacterial cells

Stripping

Release of free ammonia (NH3 ) into the atmosphere, under high pH conditions

Break-point chlorination

Conversion of ammonia into chloramines, through the addition of chlorine

Nitrate

Denitrification

Phosphate

Bacterial assimilation

Conversion of nitrate into molecular nitrogen (N2 ), which escapes into the atmosphere, under anoxic conditions Assimilation in excess of the phosphate from the liquid by phosphate accumulating organisms, which takes place when aerobic and anaerobic conditions are alternated

Precipitation

Phosphorus precipitation under conditions of high pH, or through the addition of metallic salts

Filtration

Retention of phosphorus-rich biomass, after stage of biological excessive P assimilation

Table 4.4. Treatment operations, processes and systems frequently used for the removal of pollutants from domestic sewage Pollutant Suspended solids

• • • •

Biodegradable organic matter

• • • • •

Pathogenic organisms

• • • • •

Operation, process or treatment system Screening Grit removal Sedimentation Land disposal Stabilisation ponds and variants Land disposal Anaerobic reactors Activated sludge and variants Aerobic biofilm reactors Maturation ponds Land disposal Disinfection with chemical products Disinfection with ultraviolet radiation Membranes

Overview of wastewater treatment systems

169

Table 4.4 (Continued) Pollutant Nitrogen

• • • • •

Phosphorus

• • •

Operation, process or treatment system Nitrification and biological denitrification Maturation and high-rate ponds Land disposal Physical–chemical processes Biological removal Maturation and high-rate ponds Physical chemical processes

Table 4.5. Summary description of the main biological wastewater treatment systems Facultative pond

Anaerobic pond – facultative pond

Facultative aerated lagoon

Completely mixed aerated lagoon – sedimentation pond

High rate ponds

STABILISATION PONDS Wastewater flows continuously through a pond especially constructed for wastewater treatment. The wastewater remains in the ponds for many days. The soluble and fine particulate BOD is aerobically stabilised by bacteria which grow dispersed in the liquid medium, while the BOD in suspension tends to settle, being converted anaerobically by bacteria at the bottom of the pond. The oxygen required by the aerobic bacteria is supplied by algae through photosynthesis. The land requirements are high. Around 50 to 65% of the BOD is converted in the anaerobic pond (deeper and with a smaller volume), while the remaining BOD is removed in the facultative pond. The system occupies an area smaller than that of a single facultative pond. The BOD removal mechanisms are similar to those of a facultative pond. However, oxygen is supplied by mechanical aerators instead of through photosynthesis. The aeration is not enough to keep the solids in suspension, and a large part of the sewage solids and biomass settles, being decomposed anaerobically at the bottom. The energy introduced per unit volume of the pond is high, what makes the solids (principally the biomass) remain dispersed in the liquid medium, in complete mixing. The resulting higher biomass concentration in the liquid medium increases the BOD removal efficiency, which allows this pond to have a volume smaller than a facultative aerated lagoon. However, the effluent contains high levels of solids (bacteria) that need to be removed before being discharged into the receiving body. The sedimentation pond downstream provides conditions for this removal. The sludge of the sedimentation pond must be removed every few years. High rate ponds are conceived in order to maximise algal production, in a totally aerobic environment. To accomplish this, lower depths are employed, allowing light penetration throughout the liquid mass. Therefore, photosynthetic activity is high, leading to high dissolved oxygen concentrations and pH levels. These factors contribute to the increase of the pathogens die-off and to the removal of nutrients. High rate ponds usually receive a high organic load per unit surface area. Usually a moderate agitation in the liquid is introduced, caused by a low-power mechanical equipment. (Continued )

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Wastewater characteristics, treatment and disposal

Table 4.5 (Continued) Maturation ponds

Slow rate system

Rapid infiltration

Subsurface infiltration

Overland flow

Constructed wetlands

Upflow anaerobic sludge blanket reactor (UASB)

The main objective of maturation ponds is the removal of pathogenic organisms. In maturation ponds prevail environmental conditions which are adverse to these organisms, such as ultraviolet radiation, high pH, high DO, lower temperature (compared with the human intestinal tract), lack of nutrients and predation by other organisms. Maturation ponds are a post-treatment stage for BOD-removal processes, being usually designed as a series of ponds or a single-baffled pond. The coliform removal efficiency is very high. LAND DISPOSAL The objectives may be for (a) wastewater treatment or (b) water reuse through crop production or landscape irrigation. In each case, design criteria are different. Wastewater is applied to the soil, supplying water and nutrients necessary for plant growth. Part of the liquid evaporates, part percolates into the soil, and the largest fraction is absorbed by the plants The surface application rates are very low. The liquid can be applied by sprinkling, graded-border, furrow and drip irrigation. Wastewater is applied in shallow basins. The liquid passes through the porous bottom and percolates into the soil. The evaporation loss is lower in view of the higher application rates. Vegetation may or may not be used. The application is intermittent, which provides a rest period for the soil. The most common types are: application for groundwater recharge, recovery using underdrains and recovery using wells. Pre-settled sewage (usually from septic tanks) is applied below the soil surface. The infiltration trenches or chambers are filled with a porous medium, which provides transportation, storage and partial treatment, followed by the infiltration itself. Wastewater is distributed in the upper part of vegetated slopes, flows over the slopes and is collected by ditches at the lower part. Treatment occurs in the root-soil system. The application is intermittent. Distribution of wastewater may be by high-pressure sprinklers, low-pressure sprays and gated or perforated pipes or channels. While the former systems are land-based systems, these are aquatic-based systems. The systems are composed by shallow basins or channels in which aquatic plants grow. The system can be of free-water surface (water level above ground level) or subsurface flow (water level below ground level). Biological, chemical and physical mechanisms act on the root–soil system. ANAEROBIC SYSTEMS BOD is converted anaerobically by bacteria dispersed in the reactor. The liquid flow is upwards. The upper part of the reactor is divided into settling and gas collection zones. The settling zone allows the exit of the clarified effluent in the upper part and the return of the solids (biomass) by gravity to the system, increasing its concentration in the reactor. Amongst the gases formed is methane. The system has no primary sedimentation tank. The sludge production is low, and the excess sludge wasted is already thickened and stabilised.

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Table 4.5 (Continued) Anaerobic filter

Anaerobic reactor – post-treatment

Conventional activated sludge

Activated sludge (extended aeration)

Intermittently operated activated sludge (sequencing batch reactors)

Activated sludge with biological nitrogen removal

BOD is converted anaerobically by bacteria that grow attached to a support medium (usually stones) in the reactor. The tank works submerged and the flow is upwards. The system requires a primary sedimentation tank (frequently septic tanks). The sludge production is low and the excess sludge is already stabilised. UASB reactors produce an effluent that has difficulty in complying with most existing discharge standards. Therefore, some form of post treatment is frequently necessary. The post treatment may be biological (aerobic or anaerobic) or physical-chemical (with the addition of coagulants). Practically all wastewater treatment processes may be used as a post treatment of the anaerobic reactors. The global efficiency of the system is usually similar to the one that would be obtained if the process were being applied for raw wastewater. However, land, volume and energy requirements are lower. Sludge production is also lower. ACTIVATED SLUDGE The biological stage comprises two units: aeration tank (reactor) and secondary sedimentation tank. The biomass concentration in the reactor is very high, due to the recirculation of the settled solids (bacteria) from the bottom of the secondary sedimentation tank. The biomass remains in the system longer than the liquid, which guarantees a high BOD removal efficiency. It is necessary to remove a quantity of the sludge (biomass) that is equivalent to what is produced. This excess sludge removed needs to be stabilised in the sludge treatment stage. The oxygen supply is done by mechanical aerators or by diffused air. Upstream of the reactor there is a primary sedimentation tank to remove the settleable solids from the raw sewage. Similar to the previous system, but with the difference that the biomass stays longer in the system (the aeration tanks are bigger). With this, there is less substrate (BOD) available for the bacteria, which makes them use their own cellular material as organic matter for their maintenance. Consequently, the removed excess sludge (bacteria) is already stabilised. Primary sedimentation tanks are usually not included. The operation of the system is intermittent. In this way, the reaction (aerators on) and settling (aerators off) stages occur in different phases in the same tank. When the aerators are turned off, the solids settle, which allows the removal of the clarified effluent (supernatant). When the aerators are turned on again, the settled solids return to the liquid mass, with no need of sludge recirculation pumps. There are no secondary sedimentation tanks. It can be in the conventional or extended aeration modes. The biological reactor incorporates an anoxic zone (absence of oxygen, but presence of nitrates). The anoxic zone can be upstream and/or downstream of the aerated zone. The nitrates formed in the nitrification process that takes place in the aerobic zone are used in the respiration of facultative microorganisms in the anoxic zones, being reduced to gaseous molecular nitrogen, which escapes to the atmosphere. (Continued )

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Table 4.5 (Continued) Activated sludge with biological nitrogen and phosphorus removal

Low rate trickling filter

High rate trickling filter

Submerged aerated biofilter

Rotating biological contactor (biodisc)

Besides the aerobic and anoxic zones, the biological reactor also incorporates an anaerobic zone, situated at the upper end of the tank. Internal recirculations make the biomass to be successively exposed to anaerobic and aerobic conditions. With this alternation, a certain group of microorganisms absorbs phosphorus from the liquid medium, in quantities that are much higher than those which would be normally necessary for their metabolism. The withdrawal of these organisms in the excess sludge results in the removal of phosphorus from the biological reactor. AEROBIC BIOFILM REACTORS BOD is stabilised aerobically by bacteria that grow attached to a support medium (commonly stones or plastic material). The sewage is applied on the surface of the tank through rotating distributors. The liquid percolates through the tank and leaves from the bottom, while the organic matter is retained and then further removed by the bacteria. The free spaces permit the circulation of air. In the low rate system there is a low availability of substrate (BOD) for the bacteria, which makes them undergo self-digestion and leave the system stabilised. Sludge that is detached from the support medium is removed in the secondary sedimentation tank. The system requires primary sedimentation. Similar to the previous system but with the difference that a higher BOD load is applied. The bacteria (excess sludge) need to be stabilised within the sludge treatment. The effluent from the secondary sedimentation tank is recirculated to the filter in order to dilute the influent and to guarantee a homogeneous hydraulic load. The submerged aerated biofilter is composed by a tank filled with a porous material (usually submerged), through which sewage and air flow permanently. The air flow is always upwards, while the liquid flow can be downward or upward. The biofilters with granular material undertake, in the same reactor, the removal of soluble organic compounds and particulate matter. Besides being a support medium for biomass growth, the granular material acts also as a filter medium. Periodic backwashings are necessary to eliminate the excess biomass accumulated, reducing the head loss through the medium. The biomass grows adhered to a support medium, which is usually composed by a series of discs. The discs, partially immersed in the liquid, rotate, exposing their surface alternately to liquid and air.

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Figure 4.1a. Flowsheet of stabilisation pond systems (liquid phase only).

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Figure 4.1b. Flowsheet of soil-based land treatment systems (liquid phase only).

Figure 4.1c. Flowsheet of aquatic-based land treatment systems (liquid phase only).

Figure 4.1d. Flowsheet of anaerobic reactors (liquid phase only).

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Figure 4.1e. Flowsheet of activated sludge systems (liquid phase only).

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Figure 4.1f. Flowsheet of aerobic biofilm reactors (liquid phase only).

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4.3 PRELIMINARY TREATMENT Preliminary treatment is mainly intended for the removal of:

• Coarse solids • Grit The basic removal mechanisms are of a physical order. Besides the coarse solids removal units, there is also a flow measurement unit. This usually consists of a standardised flume (e.g. Parshall flume), where the measured liquid level can be correlated with the flow. Weirs (rectangular or triangular) and closed-pipe measurement mechanisms can also be adopted. Figure 4.2 presents a typical flowsheet of the preliminary treatment.

Figure 4.2. Typical flowsheet of the preliminary treatment

The removal of coarse solids is frequently done by screens or racks, but static or rotating screens and comminutors can also be used. In the screening, material with dimensions larger than the spaces between the bars is removed (see Figure 4.3). There are coarse, medium, and fine screens, depending on the spacing between the bars. The removal of the retained material can be manual or mechanised. The main objectives of the removal of coarse solids are: • protection of the wastewater transport devices (pumps and piping) • protection of the subsequent treatment units • protection of the receiving bodies The removal of sand contained in the sewage is done through special units called grit chambers (see Figure 4.4). The sand removal mechanism is simply by sedimentation: the sand grains go to the bottom of the tank due to their larger dimensions and density, while the organic matter, which settles much slower, stays in suspension and goes on to the downstream units. There are many processes, from manual to completely mechanised units, for the removal and transportation of the settled grit. The basic purposes of grit removal are:

• • •

to avoid abrasion of the equipment and piping to eliminate or reduce the possibility of obstructions in piping, tanks, orifices, siphons, etc to facilitate the transportation of the liquid, principally the transfer of the sludge in its various phases

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Figure 4.3. Schematics of a screen

Figure 4.4. Diagram of a grit chamber

4.4 PRIMARY TREATMENT Primary treatment aims at the removal of:

• •

settleable suspended solids floating solids

After passing the preliminary treatment units, sewage still contains non-coarse suspended solids, which can be partially removed in sedimentation units. A significant part of these suspended solids is comprised of organic matter in suspension. In this way, its removal by simple processes such as sedimentation implies a reduction in the BOD load directed to the secondary treatment, where its removal is more expensive. The sedimentation tanks can be circular (Figure 4.5) or rectangular. Sewage flows slowly through the sedimentation tanks, allowing the suspended solids with a greater density than the surrounding liquid to slowly settle to the bottom. The mass of solids accumulated in the bottom is called raw primary sludge. This sludge is removed through a single pipe in small sized tanks or through mechanical scrapers and pumps in larger tanks. Floating material, such as grease and oil, tends to have a lower density than the surrounding liquid and rise to the surface of the sedimentation tanks, where they are collected and removed from the tank for subsequent treatment. The efficiency of primary treatment in the removal of suspended solids, and, as result, BOD, may be enhanced by the addition of coagulants. This is called advanced primary treatment or chemically enhanced primary treatment (CEPT).

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Figure 4.5. Schematics of a circular primary sedimentation tank

Coagulants may be aluminium sulphate, ferric chloride or other, aided or not by a polymer. Phosphorus may be also removed by precipitation. More sludge is formed, resulting from the higher amount of solids removed from the liquid and from the chemical products added. The primary sludge may be digested by conventional digesters, but in some cases it may also be stabilised by lime (simplifying the flowsheet, but further increasing the amount of sludge to be disposed of ).

Figure 4.6. Schematics of a single-chamber septic tank

Septic tanks are also a form of primary treatment (Figure 4.6). The septic tanks and their variants, such as Imhoff tanks, are basically sedimentation tanks, where the settleable solids are removed to the bottom. These solids (sludge) remain at the bottom of the tanks for a long period of time (various months) which is sufficient for their digestion. This stabilisation occurs under anaerobic conditions.

4.5 SECONDARY TREATMENT 4.5.1 Introduction The main objective of secondary treatment is the removal of organic matter. Organic matter is present in the following forms:

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•

•

181

dissolved organic matter (soluble or filtered BOD) that is not removed by merely physical operations, such as the sedimentation that occurs in primary treatment; organic matter in suspension (suspended or particulate BOD), which is largely removed in the occasionally existing primary treatment, but whose solids with slower settleability (finer solids) remain in the liquid mass.

The secondary treatment processes are conceived in such a way as to accelerate the decomposition mechanisms that naturally occur in the receiving bodies. Thus, the decomposition of the degradable organic pollutants is achieved under controlled conditions, and at smaller time intervals than in the natural systems. The essence of secondary treatment of domestic sewage is the inclusion of a biological stage. While preliminary and primary treatments have predominantly physical mechanisms, the removal of the organic matter in the secondary stage is carried out through biochemical reactions, undertaken by microorganisms. A great variety of microorganisms take part in the process: bacteria, protozoa, fungi and others. The basis of the whole biological process is the effective contact between these organisms and the organic matter contained in the sewage, in such a way that it can be used as food for the microorganisms. The microorganisms convert the organic matter into carbon dioxide, water and cellular material (growth and reproduction of the microorganisms) (see Figure 4.7). This biological decomposition of the organic matter requires the presence of oxygen as a fundamental component of the aerobic processes, besides the maintenance of other favourable environmental conditions, such as temperature, pH, contact time, etc. BACTERIAL METABOLISM NEW BACTERIA BACTERIA + ORGANIC MATTER

WATER + GASES ENERGY

Figure 4.7. Simplified diagram of bacterial metabolism

Secondary treatment generally includes preliminary treatment units, but may or may not include primary treatment units. There exists a large variety of secondary treatment processes, and the most common ones are:

• • • • •

Stabilisation ponds Land disposal systems Anaerobic reactors Activated sludge systems Aerobic biofilm reactors

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These processes have been summarised in Table 4.5, and a simplified description is presented below. In Section 4.7.2, there is a general comparison between all the processes described, including basic data (efficiencies, land requirements, power requirements, costs, sludge production, etc.), together with qualitative comparisons and a list of advantages and disadvantages.

4.5.2 Stabilisation ponds The following variants of stabilisation ponds are described briefly in this section:

• • • • • •

Facultative ponds Anaerobic pond – facultative ponds systems Facultative aerated lagoons Complete-mix aerated lagoon – sedimentation pond systems High rate ponds Maturation ponds

a) Facultative ponds Stabilisation ponds are units specially designed and built with the purpose of treating sewage. However, the construction is simple and is principally based on earth movement for digging, filling and embankment preparation. When facultative ponds receive raw sewage, they are also called primary ponds (a secondary pond would be the one which would receive its influent from a previous treatment unit, such as anaerobic ponds – see item b in this section). Amongst the stabilisation ponds systems, the process of facultative ponds is the simplest, relying only on natural phenomenon. The influent enters continuously in one end of the pond and leaves in the opposite end. During this time, which is of the order of many days, a series of events contribute to the purification of the sewage. Part of the organic matter in suspension (particulate BOD) tends to settle, constituting the bottom sludge. This sludge undergoes a decomposition process by anaerobic microorganisms and is converted into carbon dioxide, methane and other compounds. The inert fraction (non-biodegradable) stays in this bottom layer. The dissolved organic matter (soluble BOD), together with the small-dimension organic matter in suspension (fine particulate BOD), does not settle and stays dispersed in the liquid mass. Its decomposition is through facultative bacteria that have the capacity to survive, either in the presence or in the absence of free oxygen (but presence of nitrate), hence the designation of facultative, which also defines the name of the pond. These bacteria use the organic matter as energy source, which is released through respiration. The presence of oxygen is necessary in aerobic respiration, and it is supplied to the medium by the photosynthesis carried out by the algae. There is an equilibrium between consumption and the production of oxygen and carbon dioxide (see Figure 4.8).

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Bacteria  respiration:

• •

oxygen consumption carbon dioxide production

Algae  photosynthesis:

• •

oxygen production carbon dioxide consumption

Light energy O2

CO2

Aerobic zone Soluble BOD

Facultative zone

Particulate e BOD sludge layer

Influent

CO2

CH

4

H2S

Effluent

CO2 bacteria algae O2

photosynthesis

respiration

Anaerobic zone

Figure 4.8. Simplified diagram of a facultative pond

A light energy source, in this case represented by the sun, is necessary for photosynthesis to occur. For this reason, locations with high solar radiation and low cloudiness are favourable for the implementation of facultative ponds. Photosynthesis is higher near the water surface, as it depends on solar energy. Typical pond depths are between 1.5 and 2.0 m. When deep regions in the pond are reached, the light penetration is low, what causes the predominance of oxygen consumption (respiration) over its production (photosynthesis), with the possible absence of dissolved oxygen at a certain depth. Besides, photosynthesis only occurs during the day, and during the night the absence of oxygen can prevail. Owing to

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these facts, it is essential that the main bacteria responsible for the stabilisation of the organic matter are facultative, so that they can survive and proliferate, either in the presence or in the absence of oxygen (but only under anoxic, and not strict anaerobic conditions). The process of facultative ponds is essentially natural, as it does not require any equipment. For this reason, the stabilisation of the organic matter takes place at lower rates, implying the need of a long detention time in the pond (usually greater than 20 days). To be effective, photosynthesis needs a large exposure surface area to make the most of the solar energy by the algae, also implying the need of large units. As a result, the total area required by facultative ponds is the largest within all the wastewater treatment processes (excluding the land disposal processes). On the other hand, because the process is totally natural, it is associated to a high operational simplicity, which is a factor of fundamental importance in developing countries. Figure 4.9 presents a typical flowsheet of a facultative pond system.

Figure 4.9. Typical flowsheet of a facultative pond system

b) Anaerobic pond – facultative ponds systems The process of facultative ponds, in spite of having a satisfactory efficiency, requires a large area that is often not available in the locality in question. There is therefore, the need to find solutions that could imply the reduction of the total area required. One of these solutions is the system of anaerobic ponds followed by facultative ponds. In this case, the facultative pond is also called a secondary pond, since it receives the influent from an upstream treatment unit, and not the raw sewage. The raw sewage enters a pond that has smaller dimensions and is deeper (around 4 to 5 m). Owing to the smaller dimensions of this pond, photosynthesis practically does not occur. In the balance between oxygen consumption and production, consumption is much higher. Therefore, anaerobic conditions predominate in this first pond, which is consequently called an anaerobic pond. Anaerobic bacteria have a slower metabolic and reproduction rate than the aerobic bacteria. For a detention time of only 2 to 5 days in the anaerobic pond, there is only partial decomposition of the organic matter. However, the BOD removal of the order of 50 to 70%, even if insufficient, represents a large contribution, substantially reducing the load to the facultative pond that is situated downstream. The facultative pond receives a load of only 30 to 50% of the raw sewage load, which therefore allows it to have smaller dimensions. The overall area requirement

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(anaerobic + facultative pond) is such, that land savings in the order of 1/3 are obtained, compared with just a single facultative pond. The working principles of this facultative pond are exactly as described in item a. Figure 4.10 shows a typical flowsheet of a system of anaerobic ponds followed by facultative ponds.

Figure 4.10. Typical flowsheet of a system of anaerobic ponds followed by facultative ponds

The efficiency of the system is similar or only slightly higher than that of a single facultative pond. The system is also conceptually simple and easy to operate. However, the existence of an anaerobic stage in an open unit is always a cause for concern, due to the possibility of the release of malodours. If the system is well balanced, then the generation of bad smells should not occur. However, occasional operational problems could lead to the release of hydrogen sulphide, responsible for a bad smell. For this reason, this system should, whenever possible, be located far from residences. c) Facultative aerated lagoon If a predominantly aerobic system is desired, with even smaller dimensions, a facultative aerated lagoon can be used. The main difference in relation to a conventional facultative pond is regarding the form of the oxygen supply. While in facultative ponds the oxygen is mainly obtained from photosynthesis, in the case of facultative aerated lagoons the oxygen is supplied by mechanical equipment called aerators. The most commonly used mechanical aerators in aerated ponds are those with a vertical axis that rotates at a high speed, causing great turbulence in the water. This turbulence favours the penetration of atmospheric oxygen into the liquid mass, where it is then dissolved. A greater oxygen introduction is achieved, in comparison with the conventional facultative pond, which leads to a faster decomposition of the organic matter. Because of this, the detention time of the wastewater in the pond can be less (in the order of 5 to 10 days for domestic sewage). Consequently, the land requirements are much smaller. The pond is called facultative by the fact that the level of energy introduced by the aerators is only sufficient for the oxygenation, but not to maintain the solids (biomass and wastewater solids) in suspension in the liquid mass. In this way, the

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solids tend to settle and constitute a sludge layer at the bottom of the pond, to be decomposed anaerobically. Only the soluble and fine particulate BOD remains in the liquid mass, undergoing aerobic decomposition. Therefore, the pond behaves like a conventional facultative pond (see Figure 4.11).

Figure 4.11. Typical flowsheet of a system of facultative aerated lagoons

Aerated lagoons are less simple in terms of operation and maintenance, when compared with the conventional facultative ponds, owing to the introduction of mechanisation. Therefore, the reduction of the reduction of the land requirements is achieved with a certain rising in the operational level, along with the introduction of electricity consumption. d) Complete-mix aerated lagoon – sedimentation pond systems A way of reducing the aerated pond volume even further is to increase the aeration level per unit volume of the lagoon, thus creating a turbulence that, besides guaranteeing oxygenation, allows all the solids to be maintained in suspension in the liquid medium. The denomination of complete mix is because of the high degree of energy per unit volume, which is responsible for the total mixing of all the constituents in the pond. Amongst the solids maintained in suspension and in complete mixing are the biomass, besides the organic matter of the raw sewage. There is, therefore, a larger concentration of bacteria in the liquid medium as well as a larger organic matter – biomass contact. Consequently, the efficiency of the system increases and allows the volume of the aerated pond to be greatly reduced. The typical detention time in an aerated pond is in the order of 2 to 4 days. However, in spite of the high efficiency of this lagoon in the removal of the organic matter originally present in the sewage, a new problem is created. The biomass stays in suspension in all the volume and thus leaves with the pond effluent. This biomass is, in a way, also organic matter (particulate BOD), even if of a different nature of the BOD of the raw sewage. If this new organic matter were discharged into the receiving body, it would also exert an oxygen demand and cause a deterioration in the water quality. Therefore, it is important that there is a unit downstream in which the suspended solids (predominantly the biomass) can settle and be separated from the liquid

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(final effluent). This unit is a sedimentation pond, with the main purpose of permitting the settling and accumulation of the solids. The sedimentation pond is designed with short detention times, around 2 days. In this period, the solids will go to the bottom where they will undergo digestion and be stored for a period of some years, after which they will be removed. There are also sedimentation ponds with continuous removal of the bottom sludge, using, for instance, pumps mounted on rafts. The land required for this pond system is the smallest within the pond systems. The energy requirements are similar to or only slightly higher than those in the facultative aerated lagoons. However, the aspects related to sludge handling can be more complicated, due to the fact that there is a smaller storage period in the pond compared with the other systems. If the sludge is removed periodically, this will take place with an approximate frequency of around 2 to 5 years. The removal of the sludge is a laborious and expensive task. Figure 4.12 illustrates the flowsheet of the system.

Figure 4.12. Typical flowsheet of a system of complete-mix aerated lagoons – sedimentation ponds

e) High rate ponds High rate algal ponds are conceived to maximise the production of algae in a totally aerobic environment. To accomplish this, the ponds are shallow (less than 1.0 m depth), thus guaranteeing the penetration of light in all the liquid mass. Consequently, the photosynthetic activity is high, leading to high dissolved oxygen concentrations and an increase in pH (consumption of carbonic acid in the photosynthesis). These factors contribute to the increase in the death rate of the pathogenic microorganisms and the removal of nutrients, which is the main objective of the high rate ponds. Ammonia removal occurs by stripping of the free ammonia (NH3 ), since in high pH conditions the ammonia equilibrium shifts in the direction of free ammonia (with a reduction in the concentration of the ammonium ion NH+ 4 ). The increase in the NH3 concentration leads to its release to the atmosphere. Phosphorus removal also occurs as a result of the high pH, which causes the precipitation of the phosphates into the form of hydroxyapatite or struvite. The high rate ponds receive a high organic load per unit surface area. There is usually the introduction of moderate agitation in the pond, which is achieved by means of a horizontal-axis rotor or equivalent equipment. Its function is not

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to aerate, but to gently move the liquid mass. This agitation improves the contact of the influent with the bacteria and algae, reduces dead zones and facilitates the exposure of a larger quantity of algae to sun light. The configuration of the pond can be in the form of a carrousel, similar to an oxidation ditch (Figure 4.13). The high rate ponds can come after facultative ponds, in which a large part of the BOD is removed, leaving the polishing in terms of pathogen and nutrient removal for the high rate ponds. HIGH RATE POND

Figure 4.13. Schematics of a high rate pond

f) Maturation ponds Maturation ponds aim at polishing the effluent from any stabilisation pond system previously described or, in broader terms, from any sewage treatment system. The main objective of maturation ponds is the removal of pathogenic organisms and not an additional BOD removal. Maturation ponds are an economic alternative for the disinfection of the effluent, in comparison to more conventional methods, such as chlorination. The ideal environment for pathogenic microorganisms is the human intestinal tract. Outside it, whether in the sewerage system, in the sewage treatment or in the receiving body, the pathogenic organisms tend to die. Various factors contribute to this, such as temperature, solar radiation, pH, food shortage, predator organisms, competition, toxic compounds, etc. The maturation pond is designed in such a way as to optimise some of these mechanisms. Many of these mechanisms become more effective with lower pond depths, which justifies the fact that maturation ponds are shallower than the other types of ponds. Within the mechanisms associated with the pond depth, the following can be mentioned (van Haandel and Lettinga, 1994; van Buuren et al, 1995):

• • •

Solar radiation (ultraviolet radiation) High pH (pH > 8.5) High DO concentration (favouring an aerobic community, which is more efficient in the elimination of coliforms)

Maturation ponds should reach extremely high coliform removal efficiencies (E > 99.9 or 99.99% ), so that the effluent can comply with usual standards or guidelines for direct use (e.g. for irrigation) or for the maintenance of the various uses of the receiving body (e.g. bathing). The ponds also usually reach total removal of helminth eggs.

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In order to maximise the coliform removal efficiency, the maturation ponds are designed with one of the following two configurations: (a) three or four ponds in series (see Figure 4.14) or (b) a single pond with baffles.

Figure 4.14. Typical flowsheet of a system of stabilisation ponds followed by maturation ponds in series.

4.5.3 Land disposal The most common destinations for the final disposal of treated liquid effluents are water courses and the sea. However, land disposal is also a viable process, applied in various locations around the world. The land application of wastewater can be considered as a form of final disposal, of treatment (primary, secondary or tertiary level) or both. Land application of wastewater leads to groundwater recharge and/or to evapotranspiration. Sewage supplies the plants with water and nutrients. In the soil, a pollutant has basically four possible destinations:

• • • •

retention in the soil matrix retention by the plants appearance in the underground water collection by underdrains

Various mechanisms in the soil act in the removal of the pollutants:

• • •

physical (settling, filtration, radiation, volatilisation, dehydration) chemical (oxidation and chemical reactions, precipitation, adsorption, ion exchange, complexation, photochemical breakdown) biological (biodegradation and predation)

The capacity of the soil to assimilate complex organic compounds depends on its properties and on climatic conditions. Infiltration rates and types of vegetation are important factors in the use of soil as a medium for the degradation of organic compounds. These reactions require good soil aeration, which is inversely related to the humidity of the soil. Insufficient aeration leads to a lower assimilation capacity of the organic compounds by the soil. Almost all soil types are efficient in the removal of organic matter. The removal results from the filtering action of the soil, followed by the biological oxidation of the organic material. Soils with clay (fine texture) or soils with a considerable quantity of organic matter will also retain

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wastewater constituents through mechanisms of adsorption, precipitation and ion exchange. The most common types of land application are: Aquatic-based systems:

Soil-based systems: slow-rate systems rapid infiltration subsurface infiltration overland flow

• • • •

•

constructed wetlands

Aquatic-based systems (constructed wetlands) are included in this section for didactic reasons, although they could have been presented in the section of stabilisation ponds, which are also aquatic-based systems. The selection of the treatment method is a function of various factors, including required efficiency, climatic conditions, depth to ground water, soil permeability, slope etc. The application of wastewater can be done by methods such as sprinklers, furrows, graded border, drip irrigation and others. a) Slow-rate systems Depending on the design objective, slow-rate systems can be classified according to two types (WPCF, 1990):

•

•

Slow infiltration systems. Main objective: wastewater treatment. The amount of wastewater applied is not controlled by crop requirements. For municipal wastewaters, loading rates are controlled either by nitrogen loading or soil permeability. The systems are designed to maximise the amount of wastewater applied per unit land area. Crop irrigation systems. Main objective: water reuse for crop production (wastewater treatment is an additional objective). The systems are designed to apply sufficient wastewater to meet crop irrigation requirements. Loading rates are based on the crop irrigation requirement and the application efficiency of the distribution system. Nitrogen loading must be checked to avoid excess nitrogen.

In crop irrigation systems, the objective is to supply the wastewater to the soil in quantities compatible with the nutrient requirements of the crops. However, initially the microbiological and biochemical characteristics of the sewage should be evaluated, taking into consideration the type of crop, soil, irrigation system and the form in which the product will be used or consumed. Only after the verification that the sewage meets the conditions specified by the health standards should the evaluation of the chemical components be considered (Mattos, 1998). Figure 4.15 presents a flowsheet of a slow-rate system. The irrigation with wastewater can be done by flooding, furrows, sprinkler and dripping.

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Figure 4.15. Typical flowsheet of a slow-rate system

Loamy soils (medium texture) are indicated because they exhibit the best balance for wastewater renovation and drainage. The depth to the water table should be greater than 1.5 m to prevent groundwater contamination. The application rates must be compatible with the evapotranspiration of the crop in the period, therefore depending on the type of crop and the climatic conditions. In arid zones, wastewater can be used for irrigation throughout the year. On the other hand, in wet areas, the application of wastewater in rainy periods can lead to anaerobic conditions and consequently odour and insect appearance problems. Irrigation constitutes a treatment/disposal system that requires the largest surface area per unit of wastewater treated. On the other hand, it is the natural system with the highest efficiency. The plants are those mainly responsible for the removal of nutrients, such as phosphorus and nitrogen, while the microorganisms in the soil perform the removal of the organic substances. There is also a high removal of pathogenic organisms during the percolation through the soil (Mattos, 1998). b) Rapid infiltration The objective of the rapid infiltration system is to use the soil as a filtering medium for the wastewater. This system is characterised by the percolation of the wastewater, which is purified by the filtering action of the porous medium. The percolated wastewater may be used for groundwater recharge or be collected by underdrains or wells. The rapid infiltration method requires the lowest area within all the land disposal processes. Wastewater is applied in shallow infiltration basins, from which wastewater percolates through the soil. The application is intermittent, in order to allow a resting period for the soil, during which the soil dries and re-establishes aerobic conditions. Due to the higher application rates, evaporation losses are small and most of the liquid percolates through the soil, undergoing treatment. The application can be done by direct discharge (furrows, channels, perforated pipes) or by high capacity sprinklers. Vegetation growth may or may not occur,

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does not interfere with the efficiency of the process and is not part of the treatment objective (Coraucci Filho et al, 1999). Figure 4.16 presents a flowsheet of a rapid infiltration system.

Figure 4.16. Typical flowsheet of a rapid infiltration system (other types of pre-treatment may be applied)

c) Subsurface infiltration In subsurface infiltration systems, pre-settled or pre-treated sewage is applied below ground level. The infiltration sites are prepared in buried excavations, and filled in with a porous medium. The filling medium maintains the structure of the excavation, allows free sewage flow and provides sewage storage during peak flows. The sewage percolates through unsaturated soil, where additional treatment occurs. This treatment process is similar to rapid infiltration, the main difference residing in the application below ground level. The subsurface infiltration systems have the following variants:

• •

infiltration trenches or pits (without final effluent: wastewater percolates to groundwater) filtration trenches (with final effluent: collection by underdrain system)

The subsurface infiltration systems are normally used following septic tanks (Figure 4.17) and, in some cases, after further treatment provided by systems such as anaerobic filters. The applicability is usually for small residential areas or rural dwellings. d) Overland flow Overland flow systems consist of the application of untreated (at least screened) or pre-treated wastewater in the upper part of sloped terraces, planted with water resistant grasses. Wastewater flows gently downwards, having contact with the rootsoil system, in which biochemical reactions take place. Some evapotranspiration occurs, and the final effluent is collected at the lower end by drainage channels. Application is intermittent.

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Figure 4.17. Typical flowsheet of a subsurface infiltration system

The soils should have a low permeability (e.g. clay). The slope should be moderate (between 2 and 8% ). The use of vegetation is essential to increase the absorption rate of the nutrients and the loss of water by transpiration. Besides this, the vegetation represents a barrier to the free surface flow of the liquid in the soil, increasing the retention of suspended solids and avoiding erosion. This gives better conditions for the development of the microorganisms that will carry out the various biochemical reactions. The vegetation should be perennial, water tolerant grasses. Local agricultural extension agents should be consulted. Wastewater application can be done by sprinklers, gated pipes, slotted or perforated pipes or bubbling orifices (WPCF, 1990). Figure 4.18 presents a flowsheet of an overland flow system.

Figure 4.18. Typical flowsheet of an overland flow system

e) Constructed wetlands Most of the following concepts were extracted from Marques (1999), OPS/OMS (1999) and mainly EPA (2000). Natural wetlands are areas inundated or saturated by surface or groundwater that support a vegetation adapted to these conditions. The natural wetlands include marshes, swamps and similar areas, that shelter diverse forms of aquatic life.

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Constructed wetlands are purposely built wastewater treatment processes, which consist of ponds, basins or shallow canals (usually with a depth of less than 1.0 m) that shelter aquatic plants, and use biological, chemical and physical mechanisms to treat the sewage. The constructed wetlands usually have an impermeable layer of clay or synthetic membrane, and structures to control the flow direction, hydraulic detention time and water level. Depending on the system, they can contain an inert porous medium such as stones, gravel or sand. Constructed wetlands are different from natural wetlands because of human interference, such as landfills, drainage, flow alterations and physical properties. The direct use of natural wetlands for sewage treatment has great environmental impacts and must not be encouraged. There are basically two types of constructed wetlands:

•

•

Surface flow (free water surface) wetlands. These resemble natural wetlands in appearance, because they have plants which can be floating and/or rooted (emergent or submerged) in a soil layer at the bottom, and water flows freely between the leaves and the stems of these plants. There can be open areas dominated by these plants or islands exerting habitat functions. Plant genera in use include: (a) emergent: Typha, Phragmites, Scirpus, (b) submerged: Potamogeon, Elodea, etc., (c) floating: Eichornia (water hyacinth), Lemna (duckweed). Native plants are preferred. These wetlands present a very complex aquatic ecology. They may or may not have a lined bottom, depending on the environmental requirements. Water depth is between 0.6 and 0.9 m for the vegetated zones (or less, in the case of certain emergent plants), and 1.2 to 1.5 m for free water zones. This type of wetlands is adequate to receive effluent from stabilisation ponds. In these conditions, they occupy an area between 1.5 to 3.0 m2 /inhab. Subsurface flow wetlands (vegetated submersed bed systems). These do not resemble natural wetlands because there is no free water on the surface. There is a bed composed of small stones, gravel, sand or soil that gives support to the growth of aquatic plants. The water level stays below the surface of the bed, and sewage flows in contact with the roots and the rhizomes of the plants (where a bacterial biofilm is developed), not being visible or available for the aquatic biota. Plant genera that have been used are: Typha, Scirpus, Carex and Phragmites. The medium height is between 0.5 and 0.6 m and water depth is between 0.4 and 0.5 m. The gravel should have a size that allows the continuos flow of the sewage without clogging problems. A large part of the subsurface zone is anaerobic, with aerobic sites immediately adjacent to the rhizomes and roots. There is a lower potential for the generation of bad odours and the appearance of mosquitoes and rats. These wetlands are suited to receive effluents from septic tanks and anaerobic reactors, but not from stabilisation ponds, because of the presence of algae. For effluents from septic tanks, the land requirements are around 5.0 to 6.0 m2 /hab, and for effluents from anaerobic reactors, between 2.5 and 4.0 m2 /hab.

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Regarding the direction of the water flow, the wetlands can be classified as:

•

•

Vertical flow. Typically, a filter of sand or gravel planted with vegetation. At the bottom of the filter medium there is a series of underdrains that collect the treated sewage. The operation resembles the routine of a filter, with dosing and draining cycles, therefore, differing from the conventional conception of wetlands. With intermittent dosing, the flow is normally through unsaturated media. Horizontal flow. The most classical conception of constructed wetlands. May be with surface or subsurface flow.

Figure 4.19 illustrates the main variants of constructed wetlands.

Figure 4.19. Diagram showing the main variants of constructed wetlands

Constructed wetlands do not perform well in the treatment of raw sewage. Some form of primary or secondary treatment (e.g. stabilisation ponds or anaerobic reactors) must precede this process (Figure 4.20). In the case of having previous secondary treatment, low values of BOD, SS and nitrogen can be reached. The layout of wetlands is usually in cells in series or in parallel. A surface flow system receiving the effluent from a stabilisation pond can operate for 10 to 15 years without the need to remove the material composed of

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Figure 4.20. Typical flowsheets of wetlands systems

plants and inert settled solids. Of these solids, the largest part tends to accumulate at the inlet end of the unit. The operation and maintenance of constructed wetlands is very simple. Besides the activities related with the preceding treatment, the maintenance of the wetlands is usually associated with the control of undesired aquatic plants and mosquitoes (which are not normally a problem in well designed and operated subsurface flow systems). The removal of the plants is not normally necessary, but a certain pruning or replanting can be necessary to maintain the desired flow conditions and treatment.

4.5.4 Anaerobic reactors There are many variants of anaerobic reactors. This section presents only the two most widely applied for domestic sewage treatment:

• •

anaerobic filter (frequently treating septic tank effluents) UASB (upflow anaerobic sludge blanket) reactor

a) Septic tank – anaerobic filter system The system of septic tanks followed by anaerobic filters (Figure 4.21) has been widely used in rural areas and in small sized communities. The septic tanks remove most of the suspended solids, which settle and undergo anaerobic digestion at the bottom of the tank. The septic tank can be a single-chamber tank or a two-compartment tank (called an Imhoff tank). In the single chamber tank, there is no physical separation between

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the regions of the raw sewage solids sedimentation and bottom sludge digestion. The single chamber tanks can be single or in series. In the Imhoff tank, settling occurs in the upper compartment (settling compartment). The settled solids pass through an opening at the bottom of the compartment and are directed to the bottom compartment (digestion compartment). The accumulated sludge then undergoes anaerobic digestion. The gases originating from the anaerobic digestion do not interfere with the settling process, as they cannot penetrate inside the sedimentation chamber. Because septic tanks are sedimentation tanks (no biochemical reactions in the liquid phase), BOD removal is limited. The effluent, still with high organic matter concentration, goes to the anaerobic filter, where further removal takes place under anaerobic conditions. The filter is a biofilm reactor: the biomass grows attached to a support medium, usually stone. The following points are characteristic of anaerobic filters, differing from the trickling filters, which are also biofilm reactors (see Section 4.5.6):

• • • •

the liquid flow is upwards, i.e. the inlet is at the bottom and the outlet at the top of the anaerobic filter the anaerobic filter works submerged, i.e. the free spaces are filled with liquid the unit is closed the BOD load applied per unit volume is very high, which guarantees anaerobic conditions

Figure 4.21. Typical flowsheet of a system of a septic tank followed by an anaerobic filter (liquid phase)

The efficiency of a septic tank – anaerobic filter is usually less compared with fully aerobic systems, although in most situations sufficient. The system has been widely used for small populations, but there has been a trend in terms of anaerobic treatment favouring the use of anaerobic sludge blanket reactors (described below). Sludge production in anaerobic systems is very low. The excess sludge is already digested and can go directly to dewatering (in this system, typically by drying beds).

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Being an anaerobic system, there is always a risk of generation of bad odours. However, proper design and operational procedures can contribute to the reduction of these risks. It should also be remembered that the septic tank and the anaerobic reactors are closed units. b) Upflow anaerobic sludge blanket (UASB) reactors The upflow anaerobic sludge blanket (UASB) reactors are currently the main trend in wastewater treatment in some warm-climate countries, either as single units, or followed by some form of post treatment. In the UASB reactors, the biomass grows dispersed in the liquid, and not attached to a support medium, as in the case of anaerobic filters. When biomass grows it can form small granules, which are a result of the agglutination of various microorganisms. These small granules tend to serve as a support medium for other organisms. The granulation increases the efficiency of the system, but it is not essential for the working of the reactor, and is actually difficult to be obtained with domestic wastewater. The concentration of the biomass in the reactor is very high, justifying the name of sludge blanket. Owing to this high concentration, the volume required for the UASB reactor is greatly reduced in comparison with all other treatment systems. The liquid enters at the bottom, where it meets the sludge blanket, leading to the adsorption of the organic matter by the biomass. The flow is upward. As a result of the anaerobic activity, gases are formed (mainly methane and carbon dioxide) and the bubbles also present a rising tendency. The upper part of the anaerobic sludge blanket reactor presents a structure, whose functions are the separation and accumulation of the gas and the separation and return of the solids (biomass). In this way, the biomass is kept in the system (leading to high concentrations in the reactor), and only a minor fraction leaves with the effluent. This structure is called a three-phase separator, as it separates the liquid, solids, and gases. The form of the separator is frequently that of an inverted cone or pyramid. The gas is collected in the upper part of the separator, in the gas compartment, from where it can be removed for reuse (energy from methane) or burning. The solids settle in the upper part of the separator, in the settling compartment, and drain down the steeply inclined walls until they return to the reactor body. In this way, a large part of the biomass is retained by the system by simple gravitational return (differently from the activated sludge process, which requires pumping of the return sludge). Owing to the high solids retention, the hydraulic detention time can be low (in the order of 6 to 10 h). Because the gas bubbles do not penetrate the settling zone, the separation of the solids-liquid is not impaired. The effluent is relatively clarified when it leaves the settling compartment, and the concentration of the biomass in the reactor is maintained at a high level. Figure 4.22 presents a schematic view of a UASB reactor. Various configurations are possible, including circular, square or rectangular tanks. The sludge production is very low. The sludge wasted from the reactor is already digested and thickened, and may be simply dewatered in drying beds or other dewatering process. The dewaterability of the sludge is very good.

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Figure 4.22. Schematics of a UASB reactor (working principle and schematic view)

The plant flowsheet is simplified even more by the fact that, differently from anaerobic filters and other systems, there is no need for primary settling. Figure 4.23 presents the flowsheet of a wastewater treatment system comprised by a UASB reactor. The risk of generation or release of malodours can be greatly reduced by a careful design, not only in the kinetics calculations, but mainly in the hydraulic aspects. The complete sealing-off of the reactor, including a submerged exit of the effluent and the reduction of weirs, contributes noticeably to the reduction of these risks. The adequate operation of the reactor also contributes to this. A characteristic aspect of this process is the limitation in the BOD removal efficiency, which is around 70% , therefore lower than in most of the other systems. This must not be considered a disadvantage in itself, but as a characteristic of the process. To reach the desired efficiency, some form of post-treatment must follow the UASB reactors. The post-treatment process can be any of the secondary processes (aerobic or anaerobic) covered in this chapter, or a physical–chemical process, such as dissolved air flotation. The difference is that the post treatment stage is much more compact, since around 70% of the organic load has been previously removed in the anaerobic stage. Besides this, in the case of post-treatment processes that incorporate aeration, the consumption of energy is less, by virtue of the lower influent organic load to the aerated tank. Overall sludge production will be also lower. The total size (volume) of all the treatment units in the

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Figure 4.23. Typical flowsheet of a UASB reactor system (liquid phase)

UASB – post-treatment system will be slightly smaller compared with the alternative of no UASB reactor. Therefore, an economy in the construction and operation costs is usually obtained, in comparison with conventional systems not preceded by an anaerobic stage. Figure 4.24 illustrates some of the main possible combinations of UASB reactors with post-treatment systems. It can be observed that in the UASB – activated sludge and UASB – biofilm aerobic reactor systems, the aerobic biological excess sludge is simply returned to the UASB reactor, where it undergoes digestion and thickening with the anaerobic sludge, dispensing with the separate digestion and thickening units for the aerobic sludge. Thus a large simplification in the overall flowsheet is obtained, including the liquid (sewage) and solid (sludge) phases.

4.5.5 Activated sludge system There are many variants of the activated sludge process, and the present section covers only the main ones. Under this perspective, activated sludge systems may be classified according to the following categories:

•

Division according to the sludge age (see concept of sludge age in item a below): • Conventional activated sludge • Extended aeration

•

Division according to flow: • Continuous flow • Intermittent flow (sequencing batch reactors and variants)

•

Division regarding the treatment objectives: • Removal of carbon (BOD) • Removal of carbon and nutrients (N and/or phosphorus)

Figure 4.24. Examples of flowsheets of UASB reactors followed by post-treatment processes (liquid phase)

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This section presents a brief description of the main variants of the activated sludge process, which are a combination of the above divisions:

• • • • •

conventional activated sludge (continuous flow) extended aeration (continuous flow) sequencing batch reactors(intermittent operation) activated sludge with biological nitrogen removal activated sludge with biological nitrogen and phosphorus removal

All the systems above may be used as post-treatment of the effluent from anaerobic (UASB) reactors. In this case, primary sedimentation tanks (if existing) are substituted by the anaerobic reactor, and the excess sludge from the aerobic stage, if not yet stabilised, is pumped back to the anaerobic reactor, where it undergoes thickening and digestion. Biological nutrient removal is less efficient with the anaerobic pre-treatment, and adaptations or incorporation of physical-chemical treatment may be necessary. a) Conventional activated sludge When analysing the aerated pond systems described in the previous items, it becomes evident that a reduction of the volume required could be reached by increasing the biomass concentration in suspension in the liquid. The more bacteria there are in suspension, the greater the food consumption is going to be, thus the greater the assimilation of the organic matter present in the raw sewage. Within this concept, analysing the previously described aerated ponds – settling ponds system, it can be observed that there is a storage of bacteria still active in the settling unit. If part of these bacteria is returned to the aeration unit, the concentration of the bacteria in this unit will be greatly increased. This is the basic principle of the activated sludge system, in that the solids are recycled by pumping, from the bottom of the settling unit, to the aeration unit. The following items are therefore essential in the activated sludge system (liquid flow) (see Figure 4.25):

• • • •

aeration tank (reactor) settling tank (secondary sedimentation tank, also called final clarifiers) pumps for the sludge recirculation removal of the biological excess sludge

The biomass can be separated in the secondary sedimentation tank because of its property of flocculating. This is due to the fact that many bacteria have a gelatinous matrix that permits their agglutination. The floc has larger dimensions, which facilitates settling (see Figure 4.26). The concentration of the suspended solids in the aeration tank of the activated sludge system is more than 10 times greater than in a complete-mix aerated pond. The detention time of the liquid is very low, of the order of 6 to 8 hours in the conventional activated sludge system, which implies that the aeration tank volume is very small. However, owing to the sludge recirculation, the solids (biomass)

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Figure 4.25. Schematics of the units of the biological stage of the activated sludge system

Figure 4.26. Schematics of bacteria and other microorganisms forming an activated sludge floc

stay in the system for a time longer than that of the liquid. The retention time of the solids in the system is called sludge age or solids retention time, which is of the order of 4 to 10 days in the conventional activated sludge system. It is this longer retention of the solids in the system that guarantees the high efficiency of the activated sludge, as the biomass has sufficient time to metabolise practically all of the organic matter in the sewage. In the UASB reactor described in the previous section, the biomass is returned to the digestion compartment by gravity from the settling compartment situated on top of the digestion compartment and, therefore, the solids retention time is also greater than the hydraulic detention time. In the activated sludge system, the tanks are typically made of concrete, different from stabilisation ponds. To save in terms of energy for the aeration, part of the organic matter (in suspension, settleable) of the sewage is removed

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before the aeration tank, in the primary sedimentation tank. Therefore, the conventional activated sludge systems have as an integral part also the primary treatment (Figure 4.27).

Figure 4.27. Typical flowsheet of the conventional activated sludge system (liquid phase)

Owing to the continuous arrival of food in the form of BOD to the aeration tank, bacteria grow and reproduce continuously. If an indefinite population growth were allowed, the bacteria would reach excessive concentrations in the aeration tank, making the transfer of oxygen to all bacterial cells difficult. Besides this, the secondary sedimentation tank would become overloaded, the solids would not settle well and they would start to leave with the final effluent, thus deteriorating its quality. To maintain the system in equilibrium, it is necessary to draw approximately the same quantity of biomass that has increased by reproduction. This is, therefore, the biological excess sludge that can be wasted directly from the reactor or the recirculation line. The excess sludge must undergo additional treatment in the sludge treatment line. The conventional activated sludge system has low land requirements and has very good removal efficiencies. However, the flowsheet of the system is more complex than in most other treatment systems, requiring more skill for its control and operation. Energy costs for aeration are higher than for aerated ponds. b) Extended aeration In the conventional activated sludge system, the average retention time of the sludge in the system is between 4 to 10 days. With this sludge age, the biomass removed in the excess sludge still requires a stabilisation stage in the sludge treatment. This is due to the high level of biodegradable organic matter in their cell composition. However, if the biomass is retained in the system for a longer period, with a sludge age around 18 to 30 days (thus the name extended aeration), receiving the same BOD load as the conventional system, there is a lower food availability for the bacteria. Owing to the higher sludge age, the reactor has a greater volume (the liquid detention time is around 16 to 24 hours). Therefore, there is

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less organic matter per unit volume of the aeration tank, and per unit microbial mass. Consequently, in order to survive, the bacteria start to use in their metabolic processes the organic matter from their cellular material. This cellular organic matter is converted into carbon dioxide and water through respiration. This corresponds to a stabilisation (digestion) of the biomass, taking place in the aeration tank. While in the conventional system the sludge stabilisation is carried out separately (in sludge digesters in the sludge treatment line), in extended aeration systems the digestion is done concurrently with the BOD stabilisation in the reactor. As there is no need to stabilise separately the excess biological sludge, the generation of another type of sludge in the system that would require subsequent treatment is also avoided. Consequently, extended aeration systems do not usually have primary sedimentation tanks. A great simplification in the flowsheet is obtained: there are no primary sedimentation tanks and no sludge digestion units (Figure 4.28).

Figure 4.28. Flowsheet of the extended aeration system (liquid phase)

The consequence of this simplification in the system is the energy expenditure for aeration, which is due, not only to the removal of the incoming BOD, but also for the aerobic digestion of the sludge in the reactor. On the other hand, the reduction in the availability of food and its practically complete assimilation by the biomass makes extended aeration one of the most efficient sewage treatment processes in terms of BOD removal. c) Intermittent operation (sequencing batch reactors) The activated sludge systems described above are of continuous flow with relation to the sewage, that is, the sewage is always entering and leaving the reactor. However, there is a variant of the system which has intermittent flow and operation, also called a sequencing batch reactor (SBR). The principle of the activated sludge process with intermittent operation consists in the incorporation of all the units, processes and operations normally

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associated to the conventional activated sludge (primary sedimentation, biological oxidation, secondary sedimentation, sludge pumping) within a single tank. Using only a single tank, these processes and operations become sequences in time and not separated units, such as in conventional processes with continuous flow. The process of activated sludge with intermittent flow can be used in the conventional or in the extended aeration sludge ages, although the latter is more common, due to its greater operational simplicity. In the extended aeration mode, the tank also incorporates the role of the sludge digestion (aerobic) unit. The process consists of a complete-mix reactor where all the treatment stages occur. This is accomplished by the establishment of operating cycles with defined duration. The biological mass stays in the reactor, eliminating the need for separate sedimentation and sludge pumping. The retention of biomass occurs because it is not withdrawn with the supernatant (final effluent) after the sedimentation stage, remaining in the tank. The normal treatment cycle is composed of the following stages:

• • • • •

Fill (entrance of the influent in the reactor) React (aeration/mixture of the liquid/biomass contained in the reactor) Settle (sedimentation and separation of the suspended solids from the treated sewage) Draw (removal of the supernatant, which is the treated effluent from the reactor) Idle (cycle adjustment and removal of the excess sludge)

The usual duration of each stage within the cycle can be altered as a function of the influent flow variations, the treatment needs and the sewage and biomass characteristics. The wasting of excess sludge generally occurs during the last stage (idle), whose purpose is to allow the adjustment of the stages within the operating cycles of each reactor. However, as this stage is optional or may be short, sludge wasting can happen in other phases of the process. The sludge wasting quantity and frequency are established in function with the performance requirements, in the same way as in the conventional continuous flow processes. The flowsheet of the process is greatly simplified due to the elimination of various units, compared with the continuous flow activated sludge systems. The only units in an SBR operating in the extended aeration mode are: screens, grit chamber, reactors, sludge thickener (optional) and sludge drying (Figure 4.29). With domestic sewage, which arrives at the treatment plant 24 hours per day, more than one tank is necessary, since only the tank in the fill stage is apt to receive the incoming sewage. There are some variants of the sequencing batch reactor systems related to its operation (continuous feeding and discontinuous supernatant withdrawal) as well as in the sequence and duration of the stages within each cycle. These variations may lead to additional simplifications in the process or to biological nutrient removal.

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Figure 4.29. Flowsheet of a sequencing batch reactor system in the extended aeration mode (liquid phase)

d) Activated sludge with biological nitrogen removal The activated sludge system is capable of producing, without process alterations, a satisfactory conversion of ammonia to nitrate (nitrification). In this case, only ammonia and not nitrogen is removed, as there is only a conversion of the nitrogen form. Nitrification occurs almost systematically in warm-climate regions unless there is some environmental problem in the aeration tank, such as lack of dissolved oxygen, low pH, little biomass or the presence of toxic or inhibiting substances. Biological nitrogen removal is achieved in the absence of dissolved oxygen, but presence of nitrates (called anoxic conditions). In these conditions, a group of bacteria uses the nitrates in their respiration process, converting them to nitrogen gas, which escapes into the atmosphere. This process is called denitrification. To achieve denitrification in the activated sludge, process modifications are necessary, including the creation of anoxic zones and possible internal recycles. In spite of nitrogen removal being considered as tertiary treatment, biological removal is presented in this item of secondary treatment, as it consists of essentially biological processes and can be achieved through adaptations in the flowsheet of the activated sludge process at a secondary level. In activated sludge systems where nitrification occurs (mainly in warm-climate regions), it is interesting to induce denitrification to take place intentionally in the reactor. The reasons are usually associated to purely operational aspects, as well as to the final effluent quality:

•

Savings in oxygen (energy economy in the aeration). Under anoxic conditions, facultative bacteria remove BOD by using the nitrate in their respiratory processes, therefore leading to an economy of oxygen, or in other words, in the energy used for aeration. This economy partially compensates the energy expenditure for nitrification, which occurs, necessarily, under aerobic conditions.

208

•

•

•

Wastewater characteristics, treatment and disposal Savings in alkalinity (preservation of the buffering capacity). During nitrification, H+ ions are generated and alkalinity is consumed, which can lead to a decrease in the pH in the aeration tank. Conversely, denitrification consumes H+ and generates alkalinity, partially compensating the pH reduction mechanisms that occur in nitrification. Operation of the secondary sedimentation tank (to avoid rising sludge). If denitrification occurs in the anoxic conditions in the secondary sedimentation tanks, there will be a production of small nitrogen gas bubbles. These bubbles tend to adhere to the settling flocs, dragging them to the surface and causing a loss of biomass and deterioration in the final effluent quality. Nutrient control (eutrophication). The reduction of the nitrogen levels is important when the effluent is discharged into sensitive water bodies that are subjected to eutrophication (see Chapter 3).

The main process variants for nitrification and denitrification combined in a single reactor are listed below.

• • • • •

Pre-denitrification (nitrogen removal with carbon from the raw sewage) Post-denitrification (nitrogen removal with carbon from endogenous respiration) Bardenpho four-stage process Oxidation ditches Sequencing batch reactors

e) Activated sludge with biological nitrogen and phosphorus removal Although phosphorus removal can be considered as a tertiary treatment, biological removal is presented in this section on secondary treatment because it consists of essentially biological mechanisms and can be achieved through adaptations of the activated sludge process flowsheet at a secondary level. It is essential to have anaerobic and aerobic zones in the treatment line for the biological removal of phosphorus. The anaerobic zone is considered a biological selector for the phosphorus accumulating organisms. This zone allows an advantage in terms of competition for the phosphorus accumulating organisms, since they can assimilate the substrate of this zone before the other microorganisms. In this way, the anaerobic zone gives good conditions for the development or selection of a large population of phosphorus accumulating organisms in the system, which absorb substantial quantities of phosphorus from the liquid, much higher than the normal metabolic requirements. When the biological excess sludge is wasted from the system, phosphorus is removed, since it is present at high concentrations in the phosphorus accumulating organisms that are part of the withdrawn sludge. Some of the main processes used for both nitrogen and phosphorus removal in the activated sludge system are listed below. Processes employed to remove just phosphorus are not listed due to the difficulties that these undergo with the presence of nitrates in the anaerobic zone. Nitrification occurs almost systematically in the activated sludge plants in warm-climate regions. If efficient denitrification

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is not provided in the reactor, substantial quantities of nitrates are returned to the anaerobic zone through the recycle lines, impairing the maintenance of strict anaerobic conditions in the anaerobic zone. For this reason, nitrogen removal is encouraged, even if, in terms of the water body requirements, there is only the need to remove phosphorus. The literature presents a diverging nomenclature in relation to some processes, as a function of variations between commercial and scientific designations.

• • • • •

A2 O process (three-stage Phoredox) Five-stage Bardenpho process (Phoredox) UCT process Modified UCT process Sequencing batch reactors

If higher efficiencies are still desired for phosphorus removal, effluent polishing can adopted. Methods employed are:

• • •

addition of coagulants (metallic ions): phosphorus precipitation effluent filtration: removal of the phosphorus present in the suspended solids in the effluent combination of the addition of coagulants and filtration

These physical–chemical polishing methods can also be employed for P removal from other biological wastewater treatment process, and not only from the activated sludge process.

4.5.6 Aerobic biofilm reactors In this section, the aerobic units are biofilm reactors, in which the biomass grows attached to a support medium. There are many variants within this broad concept, and the following ones are presented in this section:

• • • •

Low rate trickling filter High rate trickling filters Submerged aerated biofilters Rotating biological contactors

All systems may be used as post-treatment of the effluent from anaerobic reactors. In this case, primary sedimentation tanks are substituted by the anaerobic reactor, and the excess sludge from the aerobic stage, if not yet stabilised, is pumped back to the anaerobic reactor, where it undergoes thickening and digestion. a) Low rate trickling filter A trickling filter consists of a coarse material bed, such as stones, gravel, blast furnace slag, plastic material or other, over which the wastewater is applied, in the form of drops or jets. After application, the wastewater percolates in the direction of the drains at the bottom. This percolation allows the bacterial growth on the

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surface of the support medium, forming an attached biofilm. With the passage of the wastewater, there is a contact between the microorganisms and the organic matter. The trickling filters are aerobic systems because the air circulates between the empty spaces between the stones, supplying the oxygen for the respiration of the microorganisms. The ventilation is usually natural. Wastewater is usually applied over the medium through rotating distributors, moved by the hydraulic head of the wastewater. The liquid percolates rapidly through the support medium. However, the microbial film adsorbs the organic matter, which stays adhered for a time sufficient for its stabilisation (see Figure 4.30).

Figure 4.30. Schematics of a trickling filter

The filters are normally circular and can be of various sizes in diameter (several metres). Contrary to what the name suggests, the primary function of the filter is not to filter. The diameters of the stones used are of the order of a few centimetres, which allows a large void space that is inefficient for the act of filtration by screening. The function of the medium is only to supply support for the formation of the microbial film. There are also synthetic media of various materials and forms, which present the advantage of being lighter than stone, besides having a higher surface area. However, the synthetic media are more expensive. The savings in construction costs must be analysed together with the greater expenditure in purchasing the synthetic media. With the biomass growth on the surface of the stones, the empty spaces tend to decrease, increasing the liquid velocity through the pores. When growth reaches a certain level, the velocity causes a shearing stress that dislodges part of the attached material. This is a natural form of controlling the microbial population in the medium. The dislodged sludge must be removed by the secondary sedimentation tanks to reduce the level of suspended solids in the final effluent.

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The applied BOD load per unit area and volume is lower in the low rate trickling filters. Therefore, food availability is low, which results in a partial self digestion of the sludge (self consumption of the cellular organic matter) and a higher BOD removal efficiency in the system. This is analogous to what happens in the extended aeration activated sludge system. This lower BOD load per surface unit of the tank is associated with higher area requirements when compared with high rate systems, which are described in the following item. The low rate trickling filters are still more efficient in the removal of ammonia by nitrification. The low rate system is conceptually simple. Although the efficiency of the system is comparable with the conventional activated sludge system, the operation is simpler, although less flexible. The trickling filters have a lower capacity to adjust to influent variations, besides requiring a slightly higher total area. In terms of energy consumption, the filters present a very low consumption in relation to the activated sludge system. Figure 4.31 presents a typical flowsheet of low rate trickling filters.

Figure 4.31. Typical flowsheet of a low rate trickling filter (liquid phase)

b) High rate trickling filters High rate trickling filters are conceptually similar to the low rate filters. However, because the high rate units receive a higher BOD load per unit volume of the bed, there are the following main differences: (a) the area requirements are lower; (b) there is a slight reduction in the organic matter removal efficiency; (c) sludge is not digested in the filter. Another difference is with respect to the existence of a recirculation of the final effluent. This is done with the main objectives of: (a) maintaining an approximately uniform flow during all the day (at night, the distributors could not rotate, due to the low flow, eventually drying the sludge); (b) balancing the influent load; (c) giving a new contact chance of the effluent organic matter with the biomass; and (d) bringing dissolved oxygen into the incoming liquid. The difference from the activated sludge system is that the recirculation of the high rate filters is of the liquid effluent and not of the sludge from the secondary sedimentation tanks (Figure 4.32).

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Figure 4.32. Typical flowsheet of a high rate trickling filter (liquid phase)

Another way of improving the efficiency of trickling filters or to treat wastewaters with high concentrations of organic waste is by using two filters in series. This is called a two-stage trickling filter system. There are various possible configurations with different forms of effluent recirculation. Some of the limitations of stone-bed trickling filters when operating with high organic loads refer to clogging of the void spaces, due to the excessive growth of the biofilm. In these conditions, flooding (ponding) and system failures may occur. If land availability is of concern, a careful consideration of the filter media must be exercised. The most commonly used material is still stones and gravel. However, the empty volume is limited in a trickling filter with stones, thus restricting the air circulation in the filter and consequently the quantity of oxygen available for the microorganisms and the quantity of wastewater that can be treated. The specific surface area (exposure area per unit volume of the medium) is also low, reducing the available sites for biofilm attachment and growth. To overcome these limitations, other materials can be used. These materials include corrugated plastic modules, plastic rings and others. These materials offer larger surface areas for the bacterial growth (approximately double that of typical stones), besides the significant increase in the empty spaces for air circulation. These materials are much lighter than stones (around 30 times), which allows the filters to be much higher without causing structural problems. While filters with stones are usually less than 3 metres in height, filters with synthetic media can be more than 6 metres high, substantially reducing the land required for the installation of the filters. c) Submerged aerated biofilters A submerged aerated biofilter consists of a tank filled with a porous material, through which wastewater and air permanently flow. In almost all of the existing processes, the porous medium is maintained under total immersion. The biofilter is a three-phase reactor composed of (Gon¸calves, 1996):

•

Solid phase: consists of a support medium and microorganism colonies that develop in the form of a biofilm

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• •

213

Liquid phase: consists of the liquid in permanent flow through the porous medium Gas phase: formed by artificial aeration and, in a reduced scale, by the gaseous by-products of the biological activity

The airflow in the submerged aerated biofilter is always upflow, while the liquid flow can be upflow or downflow. Biofilters with granular media remove, in the same reactor, soluble organic compounds and suspended solids from the wastewater. Besides serving as support medium for the microorganisms, the granular material performs as an effective filter. In this type of biofilter, periodic washing is necessary to eliminate the accumulated biomass, reducing the hydraulic head losses through the medium. During washing, the feeding with the wastewater is interrupted, and various sequential hydraulic discharges are made with air and cleaning water (Gon¸calves, 1996). The flowsheet of a system composed of a submerged aerated filter is presented in Figure 4.33. The two sources of sludge generation are the primary sedimentation tanks and the washing of the biofilter. The sludge from the washing is collected in a storage tank and pumped to the primary sedimentation tank for clarification outside peak flow times. Therefore, the sludge sent to the sludge treatment stage is a mixed sludge, comprising primary sludge and biological sludge (Gon¸calves, 1996). Submerged aerated biofilters are also being successfully applied for the posttreatment of UASB reactors. The aerobic sludge is returned to the UASB reactor, where it undergoes thickening and digestion, thereby simplifying substantially the overall flowsheet (see Figure 4.24) (Chernicharo et al, 2001).

Figure 4.33. Typical flowsheet of a conventional submerged aerated biofilter system (liquid phase)

Submerged aerated biofilters achieve good nitrification efficiencies and can be modified for the biological removal of nitrogen, through the incorporation of an anoxic zone in the reactor (zone below the air injection). d) Rotating biological contactors The most widely version of rotating biological contactors are the biodiscs, a process that consists of a series of spaced discs, mounted on a horizontal axis. The discs

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Wastewater characteristics, treatment and disposal

rotate slowly and maintain at each instant around half the surface immersed in the sewage and the other half exposed to the air. Biomass grows attached to the discs, forming a biofilm (see Figure 4.34). BIODISC

SURFACE EXPOSED TO AIR

IMMERSED SURFACE

Figure 4.34. Schematics of a tank with biodiscs

The discs usually are less than 3.6 metres in diameter and are generally constructed of low weight plastic. When the system is put into operation, the microorganisms of the sewage start to adhere to the rotating surfaces, where they grow until the entire disk surface is covered with a fine biological layer, a few millimetres thick. As the discs rotate, the part of the disc exposed to the air brings a thin layer of wastewater, allowing oxygen absorption through the drops and percolation on the surface of the discs. After the discs complete a rotation, this film mixes itself with the wastewater, bringing still some oxygen and mixing the partially and fully treated sewage. With the passage of the microorganisms attached to the disc surface through the wastewater, they absorb a new quantity of organic matter that is used as food. When the biological layer reaches an excessive thickness, it detaches from the discs. Part of these detached microorganisms is maintained in suspension in the liquid due to the movement of the disk, which increases the efficiency of the system. The main purposes of the discs are:

• • • •

to serve as the surface for microbial film growth; to promote the contact between the microbial film and the sewage; to maintain the biomass that detached from the discs in suspension in the liquid; to promote the aeration of the sewage that is adhered to the disc and the sewage immersed in the liquid.

The growth of the biofilm is similar in concept to the trickling filter, with the difference that the microorganisms pass through the sewage, instead of the sewage passing through the microorganisms, like in the filters. Like the trickling filter process, secondary sedimentation tanks are also necessary, with the objetive of removing the suspended solids.

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Biodisc systems are mainly used for the treatment of sewage from small communities. Due to the limitations in the diameter of the discs, it would be necessary to have a large number of discs, often impractical, for the treatment of high flows. The system presents good BOD removal efficiency, although it sometimes shows signs of instability. DO in the effluent may be high. The operational level is moderate and the construction costs are usually high. The flowsheet of the system is presented in Figure 4.35.

Figure 4.35. Typical flowsheet of a biodisc system (liquid phase)

4.6 REMOVAL OF PATHOGENIC ORGANISMS The main processes used for removal of pathogenic organisms are listed in Table 4.6. Only short comments are made, since the removal of pathogenic organisms, especially by artifical methods, is outside the scope of this book. The processes listed above are capable of reaching a coliform removal of 99.99% or more. Regarding pathogenic organisms, bacteria removal efficiency is very high (equal to or higher than coliform removal), and the other pathogens (protozoa, virus, helminths) are usually high, but variable, depending on the removal mechanism and the resistance of each species.

4.7 ANALYSIS AND SELECTION OF THE WASTEWATER TREATMENT PROCESS 4.7.1 Criteria for the analysis The decision regarding the wastewater treatment process to be adopted should be derived from a balance between technical and economical criteria, taking into account quantitative and qualitative aspects of each alternative. If the decision regarding economic aspects may seem relatively simple, the same may not be the case with the financial aspects. Besides, the technical points are in many cases intangible and in a large number of situations, the final decision can still have

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Table 4.6. Main processes for the removal of pathogenic microorganisms in sewage treatment Type

Process •

Maturation ponds

• •

Natural •

Land treatment (infiltration in soil)

•

• • •

•

•

Chlorination •

•

• •

Ozonisation •

Artificial •

Ultraviolet radiation

• • •

Comment Shallow ponds, where the penetration of solar ultraviolet radiation and unfavourable environmental conditions causes a high mortality of the pathogens. The maturation ponds do not need chemical products or energy, but require large areas. They are highly recommended systems (if there is area available), owing to their great simplicity and low costs. The unfavourable environmental conditions in the soil favour the mortality of the pathogens. In slow-rate systems, there is the possibility of plant contamination, depending on the type of application. Chemical products are not needed. Requires large areas. Chlorine kills pathogenic microorganisms (although protozoan cysts and helminth eggs are not much affected). High dosages are necessary, which may increase operational costs. The larger the previous organic matter removal, the lower the chlorine dosage required. There is a concern regarding the generation of toxic by-products to human beings. However, the great benefit to public health in the removal of pathogens must be taken into consideration. The toxicity caused by the residual chlorine in the water bodies is also of concern. The residual chlorine must have very low levels, frequently requiring dechlorination. There is much experience with chlorination in the area of water treatment in various developing countries. Ozone is a very effective agent for the removal of pathogens. Ozonisation is usually expensive, although the costs are reducing, making this alternative a competitive option in certain specific circumstances. There is less experience with ozonisation in most developing countries. Ultraviolet radiation, generated by special lamps, affects the reproduction of the pathogenic agents. Toxic by-products are not generated. Ideally, the effluent must be well clarified for the radiation to penetrate well in the liquid mass. This process has recently shown substantial development, which has made it more competitive or more advantageous than chlorination in various applications.

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Table 4.6 (Continued) Type

Process •

Membranes • •

Comment The passage of treated sewage through membranes of minute dimensions (e.g. ultrafiltration, nanofiltration) constitutes a physical barrier for the pathogenic microorganisms, which have larger dimensions than the pores. The process is highly interesting and does not introduce chemical products into the liquid. The costs are still high, but they have been reducing significantly in recent years.

subjectivity. Criteria or weightings can be attributed to the various aspects connected essentially with the reality in focus, so that the selection really leads to the most adequate alternative for the system under analysis. There are no such generalised formulas for this, and common sense and experience when attributing the relative importance of each technical aspect are essential. While the economic side is fundamental, it needs to be remembered that the best alternative is not always the one that simply presents the lowest cost in economic–financial studies. IMPORTANT ASPECTS IN THE SELECTION OF WASTEWATER TREATMENT SYSTEMS

DEVELOPED COUNTRIES

DEVELOPING COUNTRIES

Efficiency

Reliability

Sludge disposal

Land requirements

Environmental impacts

Operational costs

Construction costs

Sustainability

Simplicity

critical

important

important

critical

Figure 4.36. Critical and important aspects in the selection of wastewater treatment systems in developed and developing regions (von Sperling, 1996)

Figure 4.36 presents a comparison between important aspects in the selection of treatment systems, analysed in terms of developed and developing regions (von Sperling, 1996). The comparison is unavoidably general, owing to the specificity of each region or country and the high contrasts usually observed in developing

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countries. The items are organised in a decreasing order of importance for the developed regions. In these regions, the critical items are usually: efficiency, reliability, sludge disposal aspects and land requirements. In developing regions, these first items are organised in a similar manner of decreasing importance, but have a lower magnitude, in comparison with the developed regions. The main difference resides in what are considered the critical items for the developing regions: construction costs, sustainability, simplicity and operational costs. These items are of course important in developed regions, but cannot be usually considered critical. Table 4.7 presents general factors to be taken into account when selecting and evaluating unit operations and processes in wastewater treatment, while Table 4.8 presents environmental aspects to be considered in the selection of processes for wastewater treatment and sludge management. Each of these factors must be evaluated in terms of the local conditions and the technology employed. The reliability of the monitoring system must also be considered.

4.7.2 Comparison between the wastewater treatment systems Presented below is a comparative analysis between the main wastewater treatment systems (liquid and solid phases) applied to domestic sewage. The analysis is summarised in various tables and figures:

•

• •

•

•

• •

Quantitative comparison (Table 4.9): average effluent concentrations and typical removal efficiencies of the main pollutants of interest in domestic sewage Quantitative comparison (Table 4.10): typical characteristics of the main sewage treatment systems, expressed in per-capita values Diagrammatic comparison (Tables 4.11 to 4.13): capacity of the various sewage treatment systems in consistently reaching different quality levels in terms of BOD, COD, SS, ammonia, total nitrogen, total phosphorus, faecal coliforms and helminth eggs (based on von Sperling & Chernicharo, 2002) Diagrammatic comparison (Tables 4.14 to 4.18): per capita values of land requirement, power for aeration, production of sludge to be disposed of, construction costs and operation and maintenance costs, for various sewage treatment processes. Qualitative comparison (Table 4.19): a qualitative comparative analysis that covers various relevant aspects in the evaluation of the sewage treatment systems. The aspects of efficiency, economy, process and environmental problems are analysed. Description (Table 4.20): a list of the basic equipment usually necessary in the main sewage treatment systems. Advantages and disadvantages (Table 4.21): main advantages and disadvantages of the various sewage treatments systems. This analysis is principally oriented for the comparison of the processes within the same system, although it still permits, within certain limitations, the comparison between distinct systems.

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Table 4.7. Important factors to be considered when evaluating and selecting unit operations and processes Condition Process applicability

Applicable flow Acceptable flow variation

Influent characteristics Inhibiting or refractory compounds Climatic aspects

Process kinetics and reactor hydraulics Performance

Treatment residuals

Sludge processing

Environmental constraints

Chemical product requirements Energy requirements Requirements of other resources

Factor The applicability of a process is evaluated based on past experience, published data, data from operating works and from pilot plants. If new or unusual conditions are found, pilot scale studies are necessary. The process must be adequate for the expected flow range. The majority of the operations and processes must be designed to operate over a wide flow range. The highest efficiency is usually obtained with a constant flow, although some variation can be tolerated. Equalisation of the flow could be necessary if the variation is very large. The characteristics of the influent wastewater affect the process types to be used (e.g. chemical or biological) and the requirements for their adequate operation. What are the constituents in the wastewater that could be inhibitory or toxic, and under what conditions? What constituents are not affected during the treatment? Temperature affects reaction rates of most chemical and biological processes. Temperature can also affect the physical operation of the units. High temperatures can accelerate odour generation. The design of the reactor is based on reaction kinetics. Kinetic data are normally obtained from experience, literature or pilot studies. Reactor configuration also plays an important role in the removal of some constituents. Performance is normally measured in terms of the quality of the effluent, which should be consistent with the discharge requirements and/or the discharge standards. The type, quality and quantity of the solids, liquids and gaseous by-products need to be known or estimated. If necessary, undertake a pilot study. Are there limitations that could make the sludge processing and disposal expensive or unfeasible? What is the influence in the liquid phase of the loads recycled from the sludge treatment units? The selection of the sludge-processing system must be done in parallel with the selection of the treatment processes of the liquid phase. Environmental factors, such as prevailing winds and proximity to residential areas could restrict the use of certain processes, especially when odours are released. Noise and traffic could affect the selection of the works location. What resources and quantities must be guaranteed for the satisfactory operation of the unit for a long period of time? The energy requirements, together with the probable future energy costs, need to be estimated if it is desired to design cost-effective treatment systems. What additional resources are necessary to guarantee a satisfactory implementation and operation of the system? (Continued )

Table 4.7 (Continued) Condition Personnel requirements Operating and maintenance requirements Ancillary processes Reliability

Complexity

Compatibility Area availability

Factor How many people and what levels of skills are necessary to operate the system? Are the skills easily found? What level of training will be necessary? What are the special operational requirements that need to be provided? Which and how many spare parts will be required, and what is their availability and cost? What support processes are necessary? How do they affect the effluent quality, especially when they become inoperative? What is the reliability of the operation and process in consideration? Is the unit likely to present frequent problems? Can the process resist periodical shock loads? If yes, is the effluent quality affected? What is the complexity of the process in routine and emergency operation? What is the level of training that an operator needs to operate the process? Can the unit operation or process be used satisfactorily with the existing units? Can plant expansion be easily accomplished? Is there space availability to accommodate, not only the currently required units, but possible future expansions? Is there a buffer zone available to provide landscaping to minimise the aesthetical and environmental impacts in the neighbourhood?

Source: adapted from Metcalf & Eddy (1991)

Table 4.8. Some environmental impacts to be considered in wastewater treatment and sludge management Item Odours

Vector attraction

Noise Sludge transportation Sanitary risks

Air contamination Soil and subsoil contamination Surface or ground water contamination Devaluation of nearby areas Inconvenience to the nearby population

Comment Must be considered in the wastewater treatment and in the processing and disposal of the sludge. Important factor, mainly in urbanised areas. Vector (e.g. insects) attraction is connected with odour and can be one of the biggest problems in the sludge processing and disposal. Important factor, principally in urbanised areas. Transportation form and route need to be considered. Although difficult to be evaluated objectively, the risk is related to the number of people exposed to the sewage, receiving body and sludge, their qualities and the infection routes. Air can be contaminated by particulated material from aerosols and sprinkling. Highly variable in function of the type of wastewater treatment and sewage and sludge disposals, and the processes employed. One of the main aspects of the disposal of wastewater and sludge. Risk depends on the technology employed. The cost of land and property may be affected by the implementation of a wastewater treatment plant or a disposal site. Besides affecting many people, some solutions can generate opposition groups against the implementation of a certain system.

Source: adapted from Fernandes et al (2001)

Primary treatment (septic tanks) Conventional primary treatment Advanced primary treatment (chemically enhanced) Facultative pond Anaerobic pond + facultative pond Facultative aerated lagoon Complete-mix aerated lagoon + sedimentation pond Anaerobic pond + facult. pond + maturation pond Anaerobic pond + facultative pond + high rate pond Anaerobic pond – facultative pond + algae removal Slow rate treatment Rapid infiltration Overland flow Constructed wetlands Septic tank + anaerobic filter Septic tank + infiltration

System

400–450

400–450

150–250

120–200 120–200

120–200

120–200

100–180

100–180

100–150

30

>20 >20

>30

>30

>30

Total N (mg/L)

4

>4

3-4

4

>4

>4 >4

4

>4

Total P (mg/L)

Average quality of the effluent Helminth eggs (eggs/L) >1 >1 >1

1

1

>1

90

73–83

73–83

80–87

70–80

70–80 70–80

60–90

55–65

55–65

SS (%)

>65

>80 >65 35–65 1 >1 >1 1 >1

1 >1 >1

>1

>1

80

>80

>80

>80

>80

35–65 >80